drm/exynos: Nuke dummy fb->dirty callback

It's an optional hook. Might be needed for frontbuffer rendering on
manual upload displays, but a simple TODO doesn't explain at all what
needs to be done or why.

Cc: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>