ARM: omap2plus_defconfig: Fix missing video

When the panel-dpi driver was removed, the simple-panels driver
was never enabled, so anyone who used the panel-dpi driver lost
video, and those who used it inconjunction with simple-panels
would have to manually enable CONFIG_DRM_PANEL_SIMPLE.

This patch makes CONFIG_DRM_PANEL_SIMPLE a module in the same
way the deprecated panel-dpi was.

Fixes: 8bf4b162 ("drm/omap: Remove panel-dpi driver")

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>