drm/i915/guc/slpc: Use wrapper for reading RP_STATE_CAP

This will ensure correct values for Gen12+ platforms.

v2: Rebase

Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220216181504.7155-1-vinay.belgaumkar@intel.com