drm/amd/display: Add the MALL size in the fallback function

drm/amd/display: Add the MALL size in the fallback function

[Why & How]
If the driver has issues retrieving the MALL size for the specific
hardware, it might fail since the current value is set to zero. This
commit addresses this issue by adding a simple constant value to give
the drive a chance to start.

Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>