drm/imx: depend on COMMON_CLK to fix compile tests

The iMX DRM LVDS driver uses Common Clock Framework thus it cannot be
built on platforms without it (e.g. compile test on MIPS with RALINK and
SOC_RT305X):

/usr/bin/mips-linux-gnu-ld: drivers/gpu/drm/imx/imx-ldb.o: in function `imx_ldb_encoder_disable':
imx-ldb.c:(.text+0x400): undefined reference to `clk_set_parent'

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Philipp Zabel <[e-mail]>

drivers/gpu/drm/imx/Kconfig

···
 config DRM_IMX_LDB
 	tristate "Support for LVDS displays"
 	depends on DRM_IMX && MFD_SYSCON
+	depends on COMMON_CLK
 	select DRM_PANEL
 	help
 	  Choose this to enable the internal LVDS Display Bridge (LDB)