Select CONFIG_DRM_ETNAVIV, so that GPU support can be enabled by default.

Generated by manually selecting CONFIG_DRM_ETNAVIV and
then running:

make savedefconfig
cp defconfig arch/arm/configs/imx_v6_v7_defconfig

,which results in additional cleanup.
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>