drm/exynos: fimd: Hold pointer to driver data in context struct

This patch adds pointer to driver data to fimd_context structure, to
remove the need to call drm_fimd_get_driver_data() each time access to
driver data is necessary.

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>