The GFX/SGX functional and interface clocks have different masks, for
some unknown reason, so split EN_SGX_SHIFT into one each for fclk and
iclk.

Correct according to the TRM and the far more important 'does this
actually work at all?' metric.
Signed-off-by: Daniel Stone <daniel.stone@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>