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)

Signed-off-by: Akash Goel <akash.goel@intel.com>
Signed-off-by: Sourab Gupta <sourab.gupta@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Apply checkpatch fixup.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>