tests/kms_dither: Reset max_bpc before exiting the subtest

Before exiting the subtest, we must reset the connector's "max_bpc"
property to it's default value, else it may effect the consequent
subtests.

V2:
* Don't trigger a subtest if requested bpc > connecot's max_bpc.

Signed-off-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Reviewed-by: Karthik B S <karthik.b.s@intel.com>