drm/msm: allow compile_test on !ARM

MSM is one of the few drivers which won't even compile
test on !ARM platforms.

Looking into this a bit more it turned out that there is
actually not that much missing to at least let the driver
compile on x86 as well.

So this patch replaces the use of phys_to_page() with the
open coded version and provides a dummy for of_drm_find_bridge().

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210924071759.22659-2-christian.koenig@amd.com