drm/rockchip: unset pgoff when mmap'ing gems

Commit 371f0f08 ("ARM: 8426/1: dma-mapping: add missing range check
 in dma_mmap()") introduced offset-checking for mappings, which collides
with the fake-offset the drm sets for gems.

Other drm-drivers set this offset to 0 before doing the mapping, so
this looks like the correct way to go for rockchip as well.

Fixes: 371f0f08

 ("ARM: 8426/1: dma-mapping: add missing range check in dma_mmap()")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>