docs: gpu: i915.rst: Fix DRRS documentation

intel_drrs_enable() and intel_drrs_disable() were renamed to
intel_drrs_activate() and intel_drrs_deactivate() in commit
54903c7a

 ("drm/i915: s/enable/active/ for DRRS") and it is
causing warnings when generating the kernel documentation.

But as for a while DRRS has its own file, so here just let the tool
generate the documentation for all exported and documented functions
in intel_drrs.c.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220325183832.146472-1-jose.souza@intel.com