ARM: mxs_defconfig: use MXSFB DRM driver

Use the the DRM driver for MXSFB LCD controller (used in i.MX23/
i.MX28/i.MX6SX or i.MX7). Remove CONFIG_FB_MXS which will soon be
removed.

Note that this does not remove CONFIG_FB. CONFIG_FB gets selected
implicity by CONFIG_DRM/CONFIG_DRM_KMS_FB_HELPER.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>