drm/pl111: depend on CONFIG_VEXPRESS_CONFIG

Avoid randconfig build failures by requiring VEXPRESS_CONFIG:

aarch64-linux-gnu-ld: drivers/gpu/drm/pl111/pl111_versatile.o: in function `pl111_vexpress_clcd_init':
pl111_versatile.c:(.text+0x220): undefined reference to `devm_regmap_init_vexpress_config'

Fixes: 826fc86b

 ("drm: pl111: Move VExpress setup into versatile init")
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602215252.695994-4-keescook@chromium.org