drm/i915: fix backlight invert for non-zero minimum brightness

drm/i915: fix backlight invert for non-zero minimum brightness

When we started following the backlight minimum brightness in
745f20e2bff4 ("drm/i915: respect the VBT minimum backlight brightness")
we overlooked the brightness invert quirk. Even if we invert the
brightness, we need to take the min limit into account. We probably
missed this because the invert has only been required on gen4 for proper
operation.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101127
Fixes: 745f20e2bff4 ("drm/i915: respect the VBT minimum backlight brightness")
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170531083355.7898-1-jani.nikula@intel.com
(cherry picked from commit e9d7486eac949f2a8d121657e536c8abdd4ea088)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>