clk: msm: clock: Allow voting for gfx_mx rail via a new gpu dummy clock

Add the gpu_mx_clk to allow the graphics driver to place a vote
on the gfx_mx rail. This will be used to avoid any excess RPM
transactions during GPU NAP state.

Change-Id: Iead85657024108c1d6c6b5c2d2e04b6670b7cbb5
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>