drm/amdgpu: add flag to indicate amdgpu vm context

Flag added to indicate if the amdgpu vm context is used for compute or
graphics.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>