kms_busy: Rename tests so pipe-specific subtests are blacklistable

The blacklist for pipes E and F are not capturing kms_busy's usage of
them, because of the differing naming convention. Make that more
uniform.

Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Cc: Martin Peres <martin.peres@linux.intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>