drm/exynos: Fix build breakage on !DRM_EXYNOS_FIMD

Disabling the CONFIG_DRM_EXYNOS_FIMD (e.g. by enabling of CONFIG_FB_S3C)
leads to build error:

drivers/built-in.o: In function `exynos_dp_dpms':
binder.c:(.text+0xd6a840): undefined reference to `fimd_dp_clock_enable'
binder.c:(.text+0xd6ab54): undefined reference to `fimd_dp_clock_enable'

Fix this by changing direct call to fimd_dp_clock_enable() into optional
call to exynos_drm_crtc_ops->clock_enable(). Only the DRM_EXYNOS_FIMD
implements this op.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>