drm/pl111: make structure mode_config_funcs static

structure mode_config_funcs can be made static as it does not need to be
in global scope.  Fixes sparse warning:

warning: symbol 'mode_config_funcs' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170519111018.19641-1-colin.king@canonical.com