drm/fsl-dcu: add COMMON_CLK dependency

drm/fsl-dcu: add COMMON_CLK dependency

The fsl dcu now uses the clk-provider interfaces, which are not available
when CONFIG_COMMON_CLK is disabled:

drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.c: In function 'fsl_dcu_drm_probe':
drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.c:362:20: error: implicit declaration of function '__clk_get_name' [-Werror=implicit-function-declaration]
  pix_clk_in_name = __clk_get_name(pix_clk_in);

This adds a Kconfig dependency to prevent the driver from being enabled
in this case.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 2d701449bce1 ("drm/fsl-dcu: use common clock framework for pixel clock divider")
Acked-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462186839-2224021-1-git-send-email-arnd@arndb.de