drm/st7586: 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.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210716140801.1215-8-tzimmermann@suse.de