drm/msm: Switch to pfn mappings

I'm not entirely sure why we were using VM_MIXEDMAP.  These are never
CoW mappings.  Let's switch to be more consistent with what other
drivers and the GEM shmem helpers do.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/491218/
Link: https://lore.kernel.org/r/20220625225454.81039-2-robdclark@gmail.com