If the connected output is disabled before this test is run, then the test
will fail because actual_brightness stays at 0 instead of the value
of the desired brightness. Fix this by reading intel_backlight/device
for the output name, and enable that output at start of this test.

Changes since v1:
- Enable the connected output, instead of enabling all outputs.
Changes since v2:
- Set a dummy fb, just in case.
Changes since v3:
- Use igt_output_get_plane_type, not get_plane.

Sample failure:
IGT-Version: 1.20-g01c550a27986 (x86_64) (Linux: 4.14.0-rc3-fbconhax+ x86_64)
(pm_backlight:1203) CRITICAL: Test assertion failure function test_and_verify, file pm_backlight.c:111:
(pm_backlight:1203) CRITICAL: Failed assertion: ({ typeof(0) _a = (0); typeof(val - tolerance) _b = (val - tolerance); _a > _b ? _a : _b; }) <= result
(pm_backlight:1203) CRITICAL: error: 91200 > 0

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>