drm/ttm: cleanup ttm_page_alloc_dma.c

Remove unused defines and variables. Also stop computing the
gfp_flags when they aren't used.

No intended functional change.

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