tests/kms_atomic_transition: Fix the test execution on internal panels

As we are intended to test few cases on internal panels separately,
fix the logic to execute such tests on internal panels only.

'TRANSITION_MODESET_FAST' -> should be only for internal panels
'TRANSITION_MODESET' -> should be only for external panels

Fixes: 2527ce52 (tests/kms_atomic_transition: Convert tests to dynamic)
Cc: Karthik B S <karthik.b.s@intel.com>
Signed-off-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Reviewed-by: Karthik B S <karthik.b.s@intel.com>