drm/i915/psr: Avoid unnecessary DPCD read of DP_PSR_CAPS

intel_dp->psr_dpcd already has the required values.

Cc: Jose Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Tarun Vyas <tarun.vyas@intel.com>
Reviewed-by: Vathsala Nagaraju <vathsala.nagaraju@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180511195145.3829-4-dhinakaran.pandiyan@intel.com