drm/i915: Read the EDP DPCD and PSR Capability

v2: reuse of just created is_edp_psr and put it at right place.
v3: move is_edp_psr above intel_edp_disable
v4: remove parentheses. Noticed by Paulo.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>