drm/amdgpu: remove unused vm context flags

Remove unused AMDGPU_VM_CONTEXT_GFX and AMDGPU_VM_CONTEXT_COMPUTE
flags.

Signed-off-by: Nirmoy Das <nirmoy.das@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>