drm/i915/dsi: Retrieve max brightness level from VBT

drm/i915/dsi: Retrieve max brightness level from VBT

So far, DCS backlight driver hardcode (0xFF) for max brightness level.
MIPI DCS spec allow max 0xFFFF for set_display_brightness (51h) command.
And VBT brightness precision bits can support 8 ~ 16 bits.

We should set correct precision bits in VBT that meet panel's request.
Driver can refer to this setting then configure max brightness level
in DCS backlight driver properly.

v2: modify variable name brightness_precision_bits instead of
    max_brightness_level.
v3: fix checkpatch warning.

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Cooper Chiou <cooper.chiou@intel.com>
Cc: William Tseng <william.tseng@intel.com>
Signed-off-by: Lee Shawn C <shawn.c.lee@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210908115607.9633-5-shawn.c.lee@intel.com