drm/i915/vlv: Modifying WA 'WaDisableL3Bank2xClockGate for vlv
For disabling L3 clock gating we need to set bit 25 of MMIO
register 940c. Earlier this was being done by just writing 1
into bit 25 and resetting all other bits.
This patch modifies the routine to read-modify-write of the
register, so that the values of other bits are not destroyed.
v2: Modifying the comments and the patch commit message (Chris)