drm/gma500: Remove 2D accel code

2D acceleration is only available on PSB and MRST and very slow on both
platforms. CPU acceleration is faster so don't bother with 2D accel
anymore.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201115175420.32167-1-patrik.r.jakobsson@gmail.com