With some upcoming changes i915 might not allow
all sprite planes enabled, depending on available
bandwidth limitation. Thus the test need to decrement
amount of planes and try again, instead of panicking.

v2: Removed excessive nested conditions, making code a bit
    more readable(hopefully).

v3: Stopped using global n_planes variable as it might cause
    resources being unreleased.
    Using now parms[i].mask as a way to determine if plane
    has to be included into commit.

v4: Removed unneeded n_overlays initialization.

v5: Randomize which of sprite planes to remove if hitting
    resource limits.

v6: Replace igt_warn with igt_info, to make IGT tests happier.

v7: Removed unneeded retry logic, made plane random removal simplier.
    Great thanks to Maarten Lankhorst for suggestions.

v8: Fail if we have less than 3 planes

v9: Remove unneeded diffs

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>