drm/amd/display: reduce stack for dml32_CalculatePrefetchSchedule

Move stack variables to dummy structure.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>