drm/cirrus: Test mode against video-memory size in device-wide mode_valid

Test a display mode against the available amount of video memory in
struct drm_mode_config_funcs.mode_valid, which cirrus implements in
cirrus_mode_config_mode_valid(). This helper tests display modes against
device-wide limits. Remove the now-obsolete per-CRTC test.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215161517.5113-14-tzimmermann@suse.de