drm/amd/display: Remove unused get_max_support_fbc_buffersize

get_max_support_fbc_buffersize() is unused since 2021's
commit 94f0d0c8 ("drm/amd/display/dc/dce110/dce110_compressor: Remove
unused function 'dce110_get_required_compressed_surfacesize")
removed it's only caller.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>