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 <krzk@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>