drm/gma500: Pass struct drm_gem_object to framebuffer functions

Several framebuffer functions take a pointer to an object of type
struct gtt_range when they actually need the GEM base object. Passing
the GEM object removes some type casting and clutter.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191122100545.16812-5-tzimmermann@suse.de