clk: qcom: Add gfx3d ping-pong PLL frequency switching

The GPU clocks on msm8996 have three dedicated PLLs, MMPLL2,
MMPLL8, and MMPLL9. We leave MMPLL9 at the maximum speed (624
MHz), and we use MMPLL2 and MMPLL8 for the other frequencies. To
make switching frequencies faster, we ping-pong between MMPLL2
and MMPLL8 when we're switching between frequencies that aren't
the maximum. Implement custom rcg clk ops for this type of
frequency switching.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>