drm/i915: initialize backlight max from VBT

Normally we determine the backlight PWM modulation frequency (which we
also use as backlight max value) from the backlight registers at module
load time, expecting the registers have been initialized by the BIOS. If
this is not the case, we fail.

The VBT contains the backlight modulation frequency in Hz. Add platform
specific functions to convert the frequency in Hz to backlight PWM
modulation frequency, and use them to initialize the backlight when the
registers are not initialized by the BIOS.

v2: Fix SPT and VLV. Thanks to Clint for the VLV code.

Cc: Clint Taylor <clinton.a.taylor@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>