drm/exynos: simplify calculation of possible CRTCs

All CRTCs can only be LCD, HDMI or VIDI, so basically all CRTCs will be a
possible CRTCs. This patch removes an extra function with switch that was
only checking if the CRTC type was one of those three above.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>