ARM: imx_v6_v7_defconfig: Enable CONFIG_DRM_MSM

Since commit 2eba6907 ("drm/msm: Remove Kconfig default") the
CONFIG_DRM_MSM option is no longer selected by default on i.MX5.

Explicitly select CONFIG_DRM_MSM so that we can get GPU support
by default on i.MX51 and i.MX53.

Fixes: 2eba6907 ("drm/msm: Remove Kconfig default")
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>