drm: Add definition for eDP backlight frequency

This patch adds the following definition
- Bit mask for EDP_PWMGEN_BIT_COUNT and min/max cap
  register which only use bit 0:4
- Base frequency (27 MHz) for backlight PWM frequency
  generator.

Signed-off-by: Puthikorn Voravootivat <puthik@chromium.org>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170523223805.46372-5-puthik@chromium.org