drm/amdgpu: set the executable flag on unused Vega10 PTEs v2

Otherwise we run into a non-retry fault on access.

It seems to be a hardware bug that the executable bit has
higher priority than the valid bit.

v2: handle clears as well

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