video: mxsfb: Make sure axi clock is enabled when accessing registers

The LCDIF engines embedded in i.MX6sl and i.MX6sx SoCs need the axi clock
as the engine's system clock.  The clock should be enabled when accessing
LCDIF registers, otherwise the kernel would hang up.  We should also keep
the clock enabled when the engine is being active to scan out frames from
memory.  This patch makes sure the axi clock is enabled when accessing
registers so that the kernel hang up issue can be fixed.

Reported-by: Peter Chen <peter.chen@freescale.com>
Tested-by: Peter Chen <peter.chen@freescale.com>
Cc: <stable@vger.kernel.org> # 3.19+
Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>