drm/amdgpu: Reject impossible entities early

Currently there are two different behaviour modes when userspace tries to
operate on not present HW IP blocks. On a machine without UVD, VCE and VPE
blocks, this can be observed for example like this:

$ sudo ./amd_fuzzing --r cs-wait-fuzzing
...
amd_cs_wait_fuzzing DRM_IOCTL_AMDGPU_CTX r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_GFX r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_COMPUTE r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_DMA r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_UVD r -1
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_VCE r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_UVD_ENC r -1
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_VCN_DEC r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_VCN_ENC r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_VCN_JPEG r 0
amd_cs_wait_fuzzing AMDGPU_WAIT_CS AMD_IP_VPE r 0

We can see that UVD returns an errno (-EINVAL) from the CS_WAIT ioctl,
while VCE and VPE return unexpected successes.

The difference stems from the fact the UVD is a load balancing engine
which retains the context, so with a workaround implemented in
amdgpu_ctx_init_entity(), but which does not account for the fact hardware
block may not be present.

This causes a single NULL scheduler to be passed to
drm_sched_entity_init(), which immediately rejects this with -EINVAL.

The not present VCE and VPE cases on the other hand pass zero schedulers
to drm_sched_entity_init(), which is explicitly allowed and results in
unusable entities.

As the UVD case however shows, call paths can handle the errors, so we can
consolidate this into a single path which will always return -EINVAL if
the HW IP block is not present.

We do this by rejecting it early and not calling drm_sched_entity_init()
when there is no backing hardware.

This also removes the need for the drm_sched_entity_init() to handle the
zero schedulers and NULL scheduler cases, which means that we can follow
up by removing the special casing from the DRM scheduler.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
References: f34e8bb7 ("drm/sched: fix null-ptr-deref in init entity")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>