drm/i915: fix up a raw 64bit divide

We are seeing compilation failures on i386 in some environments due
to an undefined reference as below:

    ERROR: "__udivdi3" [drivers/gpu/drm/i915/i915.ko] undefined!

This is generated due to a raw 64 bit divide in the i915 driver.  Fix up
this raw divide.
Signed-off-by: NAndy Whitcroft <apw@canonical.com>
Acked-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_display.c

@@ -1577,7 +1577,7 @@ igdng_compute_m_n(int bytes_per_pixel, int nlanes,
 
 	temp = (u64) DATA_N * pixel_clock;
 	temp = div_u64(temp, link_clock);
-	m_n->gmch_m = (temp * bytes_per_pixel) / nlanes;
+	m_n->gmch_m = div_u64(temp * bytes_per_pixel, nlanes);
 	m_n->gmch_n = DATA_N;
 	fdi_reduce_ratio(&m_n->gmch_m, &m_n->gmch_n);