tests/kms_properties: Fix upper bound of 'max bpc' range

The available range reported by the driver is the inclusive [min_bpc,
max_bpc], so fix the upper bound used by the test accordingly. This
issue had the side-effect (on ICL for instance) of limiting 'max bpc' to
10 (instead of restoring it to the default 12) for all the tests
following kms_properties, as seen from the logs in the referenced bug
below.

Reference: https://bugs.freedesktop.org/show_bug.cgi?id=109593


Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Simon Ser <simon.ser@intel.com>