drm/vc4: Use drm_gem_fb_create()

drm_fb_cma_create() is just a wrapper around drm_gem_fb_create() now,
so use the function directly.

Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1502631125-13557-21-git-send-email-noralf@tronnes.org