drm/ttm: Remove TTM_HAS_AGP

It tries to do fancy things with excluding agp support if ttm is
built-in, but agp isn't. Instead just express this depency like drm
does and use CONFIG_AGP everywhere.

Also use the neat Makefile magic to make the entire ttm_agp_backend
file optional.

v2: Use IS_ENABLED(CONFIG_AGP) as suggested by Ville

v3: Review from Emil.

v4: Actually get it right as spotted by 0-day.

Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1459337046-25882-1-git-send-email-daniel.vetter@ffwll.ch