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>

drivers/gpu/drm/exynos/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
 config DRM_EXYNOS
 	tristate "DRM Support for Samsung SoC Exynos Series"
-	depends on OF && DRM && (ARCH_S3C64XX || ARCH_S5PV210 || ARCH_EXYNOS || ARCH_MULTIPLATFORM || COMPILE_TEST)
+	depends on OF && DRM && COMMON_CLK
+	depends on ARCH_S3C64XX || ARCH_S5PV210 || ARCH_EXYNOS || ARCH_MULTIPLATFORM || COMPILE_TEST
 	depends on MMU
 	select DRM_KMS_HELPER
 	select VIDEOMODE_HELPERS