drm/amdgpu: Unset context priority is now invalid

A context priority value of AMD_CTX_PRIORITY_UNSET is now invalid--instead of
carrying it around and passing it to the Direct Rendering Manager--and it
becomes AMD_CTX_PRIORITY_NORMAL in amdgpu_ctx_ioctl(), the gateway to context
creation.

Cc: Alex Deucher <Alexander.Deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Alex Deucher <Alexander.Deucher@amd.com>
Link: https://lore.kernel.org/r/20231017035656.8211-1-luben.tuikov@amd.com