drm/amdgpu: replace WARN with DRM_ERROR for invalid sched priority

amdgpu_sched_ioctl() currently uses WARN(1, ...) when userspace passes
an out-of-range context priority value. WARN(1, ...) is unconditional
and produces a full stack trace, which is disproportionate for a simple
input validation failure -- the invalid value is already rejected with
-EINVAL on the next line.

Replace WARN(1, ...) with DRM_ERROR() to log the invalid value at an
appropriate level without generating a stack dump. The -EINVAL return
to userspace is unchanged.

No functional change for well-formed userspace callers.

v2:
- Reworked commit message to focus on appropriate log level for
  parameter validation
- Clarified that -EINVAL behavior is preserved (Vitaly)

v3: completely drop that warning.
   Invalid parameters should never clutter the system log. (Christian)

Reviewed-by: Vitaly Prosyak <vitaly.prosyak@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Jesse Zhang <jesse.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>