drm/msm: remove qcom,gpu-pwrlevels bindings

The plan is to use the OPP bindings.  For now, remove the documentation
for qcom,gpu-pwrlevels, and make the driver fall back to a safe low
clock if the node is not present.

Note that no upstream dtb use this node.  For now we keep compatibility
with this node to avoid breaking compatibility with downstream android
dt files.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Rob Herring <robh@kernel.org>