drm/amd/display: Add NULL check for stream before dereference in 'dm_vupdate_high_irq'

Add a NULL check for acrtc->dm_irq_params.stream before
accessing its members.

Fixes below:
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:623
dm_vupdate_high_irq() warn: variable dereferenced before check
'acrtc->dm_irq_params.stream' (see line 615)

614	if (vrr_active) {
615		bool replay_en = acrtc->dm_irq_params.stream->link->replay_settings.replay_feature_enabled;
				 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
616		bool psr_en = acrtc->dm_irq_params.stream->link->psr_settings.psr_feature_enabled;
			      ^^^^^^^^^^^^^^^^^^^^^^^^^^^ New dereferences

617		bool fs_active_var_en = acrtc->dm_irq_params.freesync_config.state
618			== VRR_STATE_ACTIVE_VARIABLE;
619
620		amdgpu_dm_crtc_handle_vblank(acrtc);
621
622		/* BTR processing for pre-DCE12 ASICs */
623		if (acrtc->dm_irq_params.stream &&
		    ^^^^^^^^^^^^^^^^^^^^^^^^^^^ But the existing code assumed it could be NULL. Someone is wrong.

624		    adev->family < AMDGPU_FAMILY_AI) {
625			spin_lock_irqsave(&adev_to_drm(adev)->event_lock, flags);

Fixes: 6d31602a ("drm/amd/display: more liberal vmin/vmax update for freesync")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: ChiaHsuan Chung <chiahsuan.chung@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Ray Wu <ray.wu@amd.com>
Cc: Daniel Wheeler <daniel.wheeler@amd.com>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>