drm/amd/display: Export full brightness range to userspace

[WHY]
Userspace currently is offered a range from 0-0xFF but the PWM is
programmed from 0-0xFFFF.  This can be limiting to some software
that wants to apply greater granularity.

[HOW]
Convert internally to firmware values only when mapping custom
brightness curves because these are in 0-0xFF range. Advertise full
PWM range to userspace.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 8dbd72cb790058ce52279af38a43c2b302fdd3e5)
Cc: stable@vger.kernel.org