drm/kmb: fix array bounds warning

drm/kmb: fix array bounds warning

gcc warns about an out-of-bounds access when the using nonzero
values for 'plane_id' on kmb->plane_status:

drivers/gpu/drm/kmb/kmb_plane.c: In function 'kmb_plane_atomic_disable':
drivers/gpu/drm/kmb/kmb_plane.c:128:20: warning: array subscript 3 is above array bounds of 'struct layer_status[1]' [-Warray-bounds]
  128 |   kmb->plane_status[plane_id].ctrl = LCD_CTRL_GL2_ENABLE;
      |   ~~~~~~~~~~~~~~~~~^~~~~~~~~~
drivers/gpu/drm/kmb/kmb_plane.c:125:20: warning: array subscript 2 is above array bounds of 'struct layer_status[1]' [-Warray-bounds]
  125 |   kmb->plane_status[plane_id].ctrl = LCD_CTRL_GL1_ENABLE;
      |   ~~~~~~~~~~~~~~~~~^~~~~~~~~~
drivers/gpu/drm/kmb/kmb_plane.c:122:20: warning: array subscript 1 is above array bounds of 'struct layer_status[1]' [-Warray-bounds]
  122 |   kmb->plane_status[plane_id].ctrl = LCD_CTRL_VL2_ENABLE;

Having the array truncated to one entry seems intentional, so add
a range check before indexing it to make it clearer what is going
on and shut up the warning.

I received the warning from the kernel test robot after a private
patch that turns on Warray-bounds unconditionally.

Fixes: 7f7b96a8a0a1 ("drm/kmb: Add support for KeemBay Display")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Anitha Chrisanthus <anitha.chrisanthus@intel.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201129200927.1854104-1-arnd@kernel.org