drm/amdgpu: Add command to override the context priority.

Given a master fd we can then override the priority of the context
in another fd.

Using these overrides was recommended by Christian instead of trying
to submit from a master fd, and I am adding a way to override a
single context instead of the entire process so we can only upgrade
a single Vulkan queue and not effectively the entire process.

Reused the flags field as it was checked to be 0 anyways, so nothing
used it. This is source-incompatible (due to the name change), but
ABI compatible.

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