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.

Cc: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Ørjan Eide <orjan.eide@arm.com>
Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180130202913.28724-4-thierry.escande@collabora.com