drm/armada: use shmem helpers if possible

shmem_read_mapping_page() uses mapping_gfp_mask(mapping) as default gfp
mask. No reason to use shmem_read_mapping_page_gfp() directly if we want
the default behavior.

Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>