drm/exynos: fix to calculate CRTC shown via screen

This patch is to exactly calculate CRTC shown via screen for all cases.
Refer exynos_plane_get_size() function for this. Also source position of
fb is fixed when start position of CRTC is negative number.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>