drm/amd/display: fix ABM disablement

On recent versions of DMUB firmware, if we want to completely disable
ABM we have to pass ABM_LEVEL_IMMEDIATE_DISABLE as the requested ABM
level to DMUB. Otherwise, LCD eDP displays are unable to reach their
maximum brightness levels. So, to fix this whenever the user requests an
ABM level of 0 pass ABM_LEVEL_IMMEDIATE_DISABLE to DMUB instead. Also,
to keep the user's experience consistent map ABM_LEVEL_IMMEDIATE_DISABLE
to 0 when a user tries to read the requested ABM level.

Cc: stable@vger.kernel.org # 6.1+
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>