drm/amdgpu: extend AMDGPU_CTX_PRIORITY_NORMAL comment

Currently the AMDGPU_CTX_PRIORITY_* defines are used in both
drm_amdgpu_ctx_in::priority and drm_amdgpu_sched_in::priority.

Extend the comment to mention the CAP_SYS_NICE or DRM_MASTER requirement
is only applicable with the former.

Cc: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>