drm/msm: Drop the gpu binding

The driver currently identifies the GPU components it needs by parsing
a phandle list from the 'gpus' DT property.

This isn't the right binding to go with. So, for now, just search all
device nodes and find the gpu node we need by parsing a list of
compatible strings.

Once we know how to link the kms and gpu drivers, we'll drop this method
and use the correct binding.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>