drm/i915: quirk no PCH_PWM_ENABLE for Dell XPS13 backlight

commit e85843bec6c2ea7c10ec61238396891cc2b753a9 upstream.

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=47941
BugLink: https://bugs.launchpad.net/bugs/1163720
BugLink: https://bugs.launchpad.net/bugs/1162026
Some machines suffer from non-functional backlight controls if
BLM_PCH_PWM_ENABLE is set, so provide a quirk to avoid doing so.
Apply this quirk to Dell XPS 13 models.

Tested-by: Eric Griffith <EGriffith92@gmail.com>
Tested-by: Kent Baxley <kent.baxley@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[ kamal: backport to 3.2 ]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>