drm: shmobile: fix Kconfig dependencies

The shmobile drm driver selects BACKLIGHT_CLASS_DEVICE
as of 0a5a5499 "drm: shmobile: Add dependency on
BACKLIGHT_CLASS_DEVICE", but that option in turn depends
on BACKLIGHT_LCD_SUPPORT, so we actually have to select
both, or alternatively use 'depends on BACKLIGHT_CLASS_DEVICE'.

Further, the driver uses FB_SH_MOBILE_MERAM if that is
enabled, but this breaks if MERAM is a module while
the DRM driver is built-in. To solve this, add a dependency
on "FB_SH_MOBILE_MERAM || !FB_SH_MOBILE_MERAM", which forces
DRM_SHMOBILE to be a module if FB_SH_MOBILE_MERAM set to 'm'.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Airlie <airlied@gmail.com>