drm/amdgpu: fix incorrect MALL size for GFX1151

On GFX1151, the reported MALL cache size reflects only
half of its actual size; this adjustment corrects the discrepancy.

Signed-off-by: Tim Huang <tim.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Yifan Zhang <yifan1.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>