drm/i915/display: Remove some redundancy around CAN_PSR()

If source_support is set the platform supports PSR so no need to check
it again at every CAN_PSR().

Also removing the intel_dp_is_edp() calls, if sink_support is set
the sink connected is for sure a eDP panel.

Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209181439.215104-3-jose.souza@intel.com