radeon_vm_map_gart can use rdev->gart.pages_entry instead.

Also move the masking of the page address to radeon_vm_map_gart from its
callers.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>