drm/mipi-dbi: Use framebuffer dma-buf helpers

Replace dma_buf_begin_cpu_access() with drm_gem_fb_begin_cpu_access();
same for _end_cpu_access(). Remove some boiler-plate code. No functional
changes.

There's one left-over reference to the imported attachment that we
keep. GEM BOs with imported attachment are considered uncached and
enables special handling within the drm_fb_swab().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210716140801.1215-4-tzimmermann@suse.de