drm/i915/xehpsdv: factor out function to read RP_STATE_CAP

Instead of maintaining the same if ladder in 3 different places, add a
function to read RP_STATE_CAP.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210805163647.801064-6-matthew.d.roper@intel.com