CHROMIUM: drm/rockchip: Respect page offset for PRIME mmap calls

CHROMIUM: drm/rockchip: Respect page offset for PRIME mmap calls

When mapping external DMA-bufs through the PRIME mmap call, we might be
given an offset which has to be respected. However for the internal DRM
GEM mmap path, we have to ignore the fake mmap offset used to identify
the buffer only. Currently the code always zeroes out vma->vm_pgoff,
which breaks the former.

This patch fixes the problem by moving the vm_pgoff assignment to a
function that is used only for GEM mmap path, so that the PRIME path
retains the original offset.

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

Change-Id: Iec6e996707b0fe7e95a019423a944d98c80beaab
Signed-off-by: rjan Eide <orjan.eide@arm.com>
Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/381332
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Randy Li <randy.li@rock-chips.com>