drm/cirrus: Remove size test from cirrus_fb_create()

The DRM core implements a size check against the mode config's
limits when creating a framebuffer. [1] Remove the unnecessary
test from cirrus_fb_create() and remove the now-empty function.
Create framebuffers with drm_gem_fb_create_with_dirty().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://elixir.bootlin.com/linux/v6.1/source/drivers/gpu/drm/drm_framebuffer.c#L287

 # [1]
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215161517.5113-13-tzimmermann@suse.de