drm/i915: Move the fbdev async_schedule() into intel_fbdev.c

Reading the driver load/unload code leaves one confused as there's
an async_schedule() in the load, but not async_synchronize_full()
in sight. In fact it's hidden inside intel_fbdev.c. So let's move the
async_schedule() into intel_fbdev.c as well so that it's next to the
async_synchronize_full(), which should make the relationship easier
to see.

Plus this way we won't schedule a nop function call when fbdev is
disabled. And we were passing a pointer to a static inline
function to async_schedule(), which seems rather dubious to me.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446815313-9490-4-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>