drm/amd: Query and use ACPI backlight caps

ACPI ATIF has a function called query
backlight transfer characteristics.  Among the
information returned by this function is
the minimum and maximum input signals for the
backlight

Call that function on ACPI init.  When DM
backlight device is updated, copy over the
backlight caps into DM, but only once.  Use
the backlight caps in the backlight-to-dc
calculation

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>