clk: qcom: Add a dummy enable function for GX gdsc

Most of the time the CPU should not be touching the GX
domain on the GPU except for a very special use case when
the CPU needs to force the GX headswitch off. Add a
dummy enable function for the GX gdsc to simulate success
so that the pm_runtime reference counting is correct.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>