drm/i915: Don't use VBT for detecting DPCD backlight controls

drm/i915: Don't use VBT for detecting DPCD backlight controls

Despite the fact that the VBT appears to have a field for specifying
that a system is equipped with a panel that supports standard VESA
backlight controls over the DP AUX channel, so far every system we've
spotted DPCD backlight control support on doesn't actually set this
field correctly and all have it set to INTEL_BACKLIGHT_DISPLAY_DDI.

While we don't know the exact reason for this VBT misuse, talking with
some vendors indicated that there's a good number of laptop panels out
there that supposedly support both PWM backlight controls and DPCD
backlight controls as a workaround until Intel supports DPCD backlight
controls across platforms universally. This being said, the X1 Extreme
2nd Gen that I have here (note that Lenovo is not the hardware vendor
that informed us of this) PWM backlight controls are advertised, but
only DPCD controls actually function. I'm going to make an educated
guess here and say that on systems like this one, it's likely that PWM
backlight controls might have been intended to work but were never
really tested by QA.

Since we really need backlights to work without any extra module
parameters, let's take the risk here and rely on the standard DPCD caps
to tell us whether AUX backlight controls are supported or not. We still
check the VBT, just so we can print a debugging message on systems that
advertise DPCD backlight support on the panel but not in the VBT.

Changes since v3:
* Print a debugging message if we enable DPCD backlight control on a
  device which doesn't report DPCD backlight controls in it's VBT,
  instead of warning on custom panel backlight interfaces.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112376
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Perry Yuan <pyuan@redhat.com>
Cc: AceLan Kao <acelan.kao@canonical.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200117232155.135579-1-lyude@redhat.com