drm/i915: Use dp_to_i915 on intel_psr.c

Now that we have a generic caller let's simplify it and
clean up the intel_psr.c code a bit.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180827223021.7145-2-rodrigo.vivi@intel.com