video: mx3fb: Allow blocking during framebuffer allocation

No need to allocate the framebuffer from the atomic pool, we are not
in interrupt context. Adding GFP_KERNEL to the framebuffer allocation
allows to use the much bigger CMA pool to allocate the framebuffer.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-fbdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>