drm/i915: Change i915.enable_psr parameter to use per platform default.

This will give us flexibility to enable PSR by default independently so
issues and corner cases in one platform won't affect others were we have
it working properly.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>