[SRU][pqrs] drm/i915: i915.disable_pch_pwm overrides PCH_PWM_ENABLE quirk

[Kamal Mostafa]

BugLink: https://bugs.launchpad.net/bugs/1163720
BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=47941

[Impact]

It has been determined that on the Dell XPS13, enabling some UEFI options
causes backlight problems since the introduction of
  e85843b ("drm/i915: quirk no PCH_PWM_ENABLE for Dell XPS13 backlight")
which exists upstream and in ubuntu-[pqrst].

Depending on the model of XPS13 and the UEFI configuration, some systems
experience black-screen on boot with no way to increase brightness; others
just lose brightness controls.  The exact matrix of configurations and
behaviors is not well understood.

This patch provides a boot parameter to allow XPS13 UEFI users to override
the change introduced by e85843b, restoring the (reported good) behavior
they had before while still retaining the beneficial fix for the standard
BIOS XPS13 users.

This patch was submitted upstream, but they've chosen to look into a
long-term solution to this and other problems involving backlight vs. UEFI
instead.  That solution (when it lands) may not be back-portable to our
earier releases.


[Test Case]

Boot with i915.disable_pch_pwm=0 on an affected XPS13 with UEFI.  Observe
that backlight is no longer stuck at minimum on boot.  Brightness controls
may also be restored by this, depending on UEFI settings.

The patch has been tested by several users.


[Regression Potential]

Very low.  This patch does not change any behavior by default (only does
when the boot param is used), and only targets Dell XPS13 models.


[Other info]

For the time being, I'd like to apply this fix to ubuntu-[pqrs], but wait
for the upstream fix to land in ubuntu-trusty.