drm/gma500/mmu: Actually return an error if one occurs

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/gma500/mmu.c: In function ‘psb_mmu_insert_pfn_sequence’:
 drivers/gpu/drm/gma500/mmu.c:651:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115181601.3432599-7-lee.jones@linaro.org