drm/i915: Add dev_priv->psr_mmio_base

Drop the EDP_PSR_BASE() thing, and just stick the PSR register offset
under dev_priv, like we for DSI and GPIO for example.

TODO: could probably move a bunch of this kind of stuff into the device
info instead...

v2: Drop the spurious whitespace change (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447266856-30249-7-git-send-email-ville.syrjala@linux.intel.com