exynos: introduce g2d_add_base_addr helper function

In almost all functions the base address register is written, so it
makes sense to have a helper function for this.

v3: Wrap line as pointed out by Emil Velikov
    <emil.l.velikov@gmail.com>.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Tested-by: Joonyoung Shim <jy0922.shim@samsung.com>