drm/exynos: clean up description of exynos_drm_crtc

This patch removes unnecessary descriptions on
exynos_drm_crtc structure and adds one description
which specifies what pipe_clk member does.

pipe_clk support had been added by below patch without any description,
	 drm/exynos: add support for pipeline clock to the framework
		Commit-id : f26b9343

Signed-off-by: Inki Dae <inki.dae@samsung.com>