drm/i915/psr: Check if sink PSR capability changed

drm/i915/psr: Check if sink PSR capability changed

eDP specification states that sink can have its PSR capability
changed, I have never found any panel doing that but lets add that
for completeness.
For now it is not reading back the PSR capabilities and if possible
re-enabling PSR, this will be added if a panel is found using this
feature.

v4:
Cleaning DP_PSR_CAPS_CHANGE

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191128014852.214135-4-jose.souza@intel.com