Doing modeset on internal panels may have a considerable overhead due to
the panel specific power sequencing delays. To avoid long test runtimes
in CI split out the testing of internal panels from the plane modeset
subtests and test only a reduced number of plane combinations on these:
where only a single plane is enabled, all planes are disabled or all
planes are enabled.

v2:
- Add code comment explaining the need for the separate internal-panels
  subtests.
- Calculate the number of enabled planes only once. (Chris)
- Disable the slow internal-panels subtests testing all combinations.
  (Petri)

v3: (Maarten)
- Skip the internal-panels subtests if no internal panels are found.
- Also test transitions with all planes disabled.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103334


Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Petri Latvala <petri.latvala@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>