drm/i915: Enable fastset for non-boot modesets.

Now that our state comparison functions are pretty complete, we should
enable fastset by default when a modeset can be avoided. Even if we're
not completely certain about the inherited state, we can be certain
after the first modeset that our sw state matches the hw state.

There is one testcase explicitly testing fastset,
kms_panel_fitting.atomic-fastset but other testcases do so indirectly
because most tests don't clean up the display during exit, or otherwise
indirectly preserve mode by doing igt_display_reset or inheriting during
init.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
[mlankhorst: Use DRM_DEBUG_KMS. (j4ni)]
Link: https://patchwork.freedesktop.org/patch/msgid/20190108160842.13396-3-maarten.lankhorst@linux.intel.com