drm/i915: panel: invert brightness via parameter

Following the documentation of the Legacy Backlight Brightness (LBB)
Register in the configuration space of some Intel PCI graphics adapters,
setting the LBB register with the value 0x0 causes the backlight to be
turned off, and 0xFF causes the backlight to be set to 100% intensity
(http://download.intel.com/embedded/processors/Whitepaper/324567.pdf

).
The Acer Aspire 5734Z, however, turns the backlight off at 0xFF and sets
it to maximum intensity at 0. In consequence, the screen of this systems
becomes dark at an early boot stage which makes it unusable. The same
inversion applies to the BLC_PWM_CTL I915 register. This problem was
introduced in kernel version 2.6.38 when the PCI device of this system
was first supported by the i915 KMS module.

This patch adds a parameter to the i915 module to enable inversion of
the brightness variable (i915.invert_brightness).

Signed-off-by: Carsten Emde <C.Emde@osadl.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>