i965/screen: Allow drirc to set 'allow_rgb10_configs' again.

i965/screen: Allow drirc to set 'allow_rgb10_configs' again.

Since setup of ALLOW_RGB10_CONFIGS was moved to i965's own
brw_config_options.xml, this was hard-coded to false and
could not be overriden by drirc. Add some parsing into
i965's private screen->optionCache to enable drirc again.

Fixes: b391fb26df9f1b ("dri_util: remove ALLOW_RGB10_CONFIGS option (v2)")
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Marek Olšák <marek.olsak@amd.com>
Cc: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>