drm/exynos: remove superfluous checks in g2d_check_reg_offset()

The cases of the switch statement ensure that reg_type
can never be REG_TYPE_NONE here.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>