drm/xe/display: read PCON capability only when present

drm/xe/display: read PCON capability only when present

Avoid reading the PCON capabilities redundantly on non-branch devices.

v2:
 - Make commit description more accurate. (Imre)
 - Clear intel_dp->pcon_dsc_dpcd irrespective of presense of PCON. (Imre)

Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/20250619042629.3980244-1-chaitanya.kumar.borah@intel.com