msm: kgsl: Allow bus_min/_max to be set in dtsi files

Right now these values are set by reading the bus table and depending
on the OCMEM bandwidth vote corresponding to the GPU frequency.  This
method is still supported, but will be overridden if the values are
specified in the dtsi file.

Change-Id: Ic81e90be6cd63b314ac19e5f78f1d4dcbd253994
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>