CHROMIUM: drm/rockchip: Respect page offset in IOMMU mmap

CHROMIUM: drm/rockchip: Respect page offset in IOMMU mmap

When mapping buffers through the PRIME DMA-buf mmap path we might be
given an offset which has to be respected. The DRM GEM mmap path already
takes care of zeroing out the fake mmap offset, so we can just make the
IOMMU mmap implementation always respect the offset.

BUG=chrome-os-partner:56615
TEST=graphics_GLBench

Change-Id: Iec83e720b24ddd35a92f3df8312015bc5af798f0
Signed-off-by: rjan Eide <orjan.eide@arm.com>
Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/386477
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Randy Li <randy.li@rock-chips.com>