drm/gma500: Inline psbfb_create() into psbfb_probe()

Inline psbfb_create() into its only caller psbfb_probe(). Streamline
the color-depth selection. Also clean up the naming around struct
drm_fb_helper_funcs.

v2:
	* rename psbfb_probe() (Patrik)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230313151610.14367-6-tzimmermann@suse.de