drm/exynos: fimd: Add support for FIMD variants with clock selection

Some platforms that can be supported this driver has additional clock
source selection bits in VIDCON0 register that allows to select which
clock should be used to drive the pixel clock: bus clock or special
clock.

Since this driver assumes that special clock always drives the pixel
clock, this patch sets the selection bitfield to use the special clock.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Acked-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>