drm/arc: 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: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1502631125-13557-5-git-send-email-noralf@tronnes.org