efifb: Enable write-combining

efifb: Enable write-combining

Running fbcon on an uncached framebuffer is remarkably slow.  So try
to enable write combining in efifb.

Without this patch, it takes 5.8 seconds from efifb probe to i915
probe (default options; no plymouth or quiet mode).  With this patch,
it only takes 1.7 seconds.  That means we wasted over 4 seconds just
writing to UC memory.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>