drm/amdgpu: fix MES GFX mask

Current MES GFX mask prevents FW to enable oversubscription. This patch
does the following:
- Fixes the mask values and adds a description for the same
- Removes the central mask setup and makes it IP specific, as it would
  be different when the number of pipes and queues are different.

v2: squash in fix from Shashank

Cc: Christian König <Christian.Koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Arvind Yadav <arvind.yadav@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>