drm/amdgpu/uapi: Clarify comment on AMDGPU_VM_PAGE_PRT

drm/amdgpu/uapi: Clarify comment on AMDGPU_VM_PAGE_PRT

In the context of the amdgpu uAPI, the PRT flag is referring only
to unmapped pages of a partially resident texture (aka. sparse
resource), but not the full resource.

Virtual addresses marked with this flag behave as follows:
- Reads return zero
- Writes are discarded

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>