staging/sm750fb: Declare fb_ops as constant

Split up lynxfb_ops and declare each as constant. The fb_ops
instance used to be modified while initializing the driver. It is
now constant and the driver picks the correct instance, depending
on the settings for acceleration and cursor support.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: Teddy Wang <teddy.wang@siliconmotion.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-staging@lists.linux.dev
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231127131655.4020-26-tzimmermann@suse.de