drm: mxsfb_crtc: Reset the eLCDIF controller

According to the eLCDIF initialization steps listed in the MX6SX
Reference Manual the eLCDIF block reset is mandatory.

Without performing the eLCDIF reset the display shows garbage content
when the kernel boots.

In earlier tests this issue has not been observed because the bootloader
was previously showing a splash screen and the bootloader display driver
does properly implement the eLCDIF reset.

Add the eLCDIF reset to the driver, so that it can operate correctly
independently of the bootloader.

Tested on a imx6sx-sdb board.

Cc: <stable@vger.kernel.org>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1494007301-14535-1-git-send-email-fabio.estevam@nxp.com