gfx: display: initialize backlight PWM frequency

gfx: display: initialize backlight PWM frequency

Some DV1s, and DV1.5s and later use I2C to adjust the backlight. We have
PWM based backlight control enabled for compatibility with older, non-CABC
devices. The PWM clock divider was not initialized, causing the PWM signal
to run at the base clock frequency of 19.2 MHz.

Initialize the clock divider, setting the PWM frequency to 9600 Hz. The
panel specification has no recommendation on the frequency, but the bug
ANDROID-1342 has this experimental value.

Issue: ANDROID-1342
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>