drm/amdgpu: cleanup PTE flag generation v3

Move the ASIC specific code into a new callback function.

v2: mask the flags for SI and CIK instead of a BUG_ON().
v3: remove last missed BUG_ON().

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