drm/gma500: Remove CONFIG_X86 conditionals from source files

Remove the CONFIG_X86 conditionals from the source code. The driver
already depends on X86 in the Kconfig file. Also, no one has been
trying to build it on a non-x86 platform recently, or they would have
noticed that drm_ttm_cache_flush() doesn't exist.

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/20210129095604.32423-5-tzimmermann@suse.de