drm/gma500: Fix naming in fb_ops

Fix some names around struct fb_ops to better fit the overall naming
conventions. No functional changes.

v2:
	* fix typo in commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230313151610.14367-5-tzimmermann@suse.de