drm/pl111: select DRM_PANEL

When DRM_PANEL is disabled, we get a link error for pl111:

drivers/gpu/built-in.o: In function `pl111_connector_destroy':
pl111_connector.c:(.text+0x3487e6): undefined reference to `drm_panel_detach'

For some reason this only appears in the latest linux-next
although the driver appears to have used the symbol for a few
weeks already. The solution however is simple enough, we just
need to add a 'select' statement.

Fixes: bed41005 ("drm/pl111: Initial drm/kms driver for pl111")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170530092224.1204037-1-arnd@arndb.de