Plug in the bdw+ irq handling for PSR interrupts. bdw+ supports psr on
any transcoder in theory, though the we don't currenty enable PSR except
on the EDP transcoder.

v2: From DK
 * Rebased on drm-tip
v3: Switched author to Ville based on IRC discussion.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Jose Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180403212420.25007-2-dhinakaran.pandiyan@intel.com