drm: shmobile: Reduce include dependencies

This file doesn't need anything provided by <linux/kernel.h>.
All it needs are some types, which are provided by <drm/drm_mode.h>.

Drop unneeded <linux/kernel.h> completely.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200422125201.37618-1-andriy.shevchenko@linux.intel.com