clk: msm: clock: Add support for programming the GCC_GPU_IREF_EN register

Add a new gcc_gpu_iref_clk that the graphics driver can control
as needed. The default state of the clock is ON; so having this
control will mean saving current.

CRs-Fixed: 1024948
Change-Id: I562bb546f49b1605f20fb7d705f40584d190230b
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>