drm/amdgpu/display: re-add surface size calculation in dcn30_hwseq.c

This is required for MALL.  Was accidently removed in PSR update.

Fixes: 48e48e59 ("drm/amd/display: Disable idle optimization when PSR is enabled")
Fixes: 52f2e83e ("drm/amdgpu/display: add MALL support (v2)")
Acked-by: Slava Abramov <slava.abramov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>