drm/i915: Auto detect DPCD backlight support by default

Turns out we actually already have some companies, such as Lenovo,
shipping machines with AMOLED screens that don't allow controlling the
backlight through the usual PWM interface and only allow controlling it
through the standard EDP DPCD interface. One example of one of these
laptops is the X1 Extreme 2nd Generation.

Since we've got systems that need this turned on by default now to have
backlight controls working out of the box, let's start auto-detecting it
for systems by default based on what the VBT tells us. We do this by
changing the default value for the enable_dpcd_backlight module param
from 0 to -1.

Tested-by: AceLan Kao <[email protected]>
Tested-by: Perry Yuan <[email protected]>
Signed-off-by: Lyude Paul <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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