dri/common: Add option to allow exposure of 10 bpc color configs. (v2)

dri/common: Add option to allow exposure of 10 bpc color configs. (v2)

Some clients may not like RGB10X2 and RGB10A2 fbconfigs and
visuals. Add a new driconf option 'allow_rgb10_configs' to
allow per application enable/disable.

The option defaults to enabled.

v2: Rename expose_rgb10_configs to allow_rgb10_configs,
    as suggested by Emil. Add comment to option parsing,
    to make sure it stays before the ->InitScreen().

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>