Testcase for plane visibility after atomic modesets. The idea of the test
is the following:

 - draw a blue screen with high resolution
 - enable a yellow plane, visible, in lower-left corner
 - set a new lower resolution mode (1024x768) that makes plane invisible
 - check from debugfs 'i915_display_info' that the plane is invisible
 - switch back to higher resolution mode
 - check from debugfs 'i915_display_info' that the plane is visible again
 - repeat number of iterations, default 64

v2: allow test to be run on non-Intel drivers (Daniel)
    moved test for plane visibility to as helper function (Daniel)
    moved get_vblank() function to be part of helper functions (Daniel)
    rename 'tiling' parameter as 'modifier' (Daniel)
    select a mode from a list so that the plane should be invisible.
    use default 1024x768 mode only as a fallback if decent mode has not
    been found (Daniel)
    add tiling MODE_NONE (Daniel)

v3: draw as many overlay planes as the platform supports + cursor plane
    on top of each other on lower-left corner
    skip the test if i915_display_info file is not available
    test plane visibility with igt_assert_plane_visibility() function
    drop option for multiple test iterations (Daniel Vetter)

v4: switch 'for_each_connected_output()' to
    'for_each_valid_output_on_pipe()'
    skip Y and Yf tiling for generations older than 9 (Maarten)

Cc: Daniel Stone <daniel@fooishbar.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>