drm/gma500: Reimplement psb_gem_create()

Implement psb_gem_create() for general use. Create the GEM handle in
psb_gem_create_dumb(). Allows to use psb_gem_create() for creating all
of the GEM objects.

While at it, clean-up drm_gem_dumb_create() to make it more readable.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015084053.13708-4-tzimmermann@suse.de