sgxpvr: Don't ignore interrupts when preparing to sleep.

Setting the "ignore interrupts" (aka powering_down) mask too early in
sleeping preparation hits the assertion later on when the code tries
to wait for interrupt acknowledging command reception.
This also stops kernel complaints about unhandled interrupts from
the gfx.
Move the setting to just before we really power down the gfx cores.

Change-Id: I5451cca3573bde0a37687ebd08b039e45946652c
Merge "sgxpvr: Don't ignore interrupts when preparing to sleep." into cm-10.2