drm/i915: Mask LPSP to get PSR working even with Power Well in use by audio.

Power Well in use forces constantly PSR to exit.
On recent Kernel I noticed that PSR Performance Counter was always 0
indicating that PSR was never really achieved.
By masking LPSP, PSR can work normally and save power on Haswell.

Two bugs had been raised with PSR flag enabled:
- "Screen flickers when booted by enabling PSR in the kernel (i915.enable_psr=1) , the system is booting to a gray screen."
- "When booting the DUT with PSR feature enabled in the kernel (i915.enable_psr=1) , the system is booting to a gray screen."

Both bugs has been fixed by this patch.

v2: proper comment for -fixes

Tested-by: Selvaraj, Elavarasan <elavarasanx.selvaraj@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>