drm/gma500: Never wait for blits

Blits cannot happen anymore since we removed the 2d accel code. Stop
checking for a busy blitter and remove the remaining blitter code.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210201132617.1233-6-patrik.r.jakobsson@gmail.com