drm/exynos: depend on COMMON_CLK to fix compile tests

drm/exynos: depend on COMMON_CLK to fix compile tests

The Exynos DRM 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/exynos/exynos_mixer.o: in function `mixer_bind':
    exynos_mixer.c:(.text+0x958): undefined reference to `clk_set_parent'

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>