drm/exynos: fimd: Add support for FIMD versions without SHADOWCON register

Some platforms that can be supported with this driver have PRTCON
register instead of SHADOWCON, which requires slightly different
handling.

This patch factors out all register shadow control code from the driver
and adds a function to control register shadowing appropriately,
depending on driver data.

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>