drm/amd/display: fix calc_pll_max_vco_construct

This was broken by a previous change switching to cached fw_info.
Fixed by inverting a valid bool check.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=111432


Fixes: 9adc8050 ("drm/amd/display: make firmware info only load once during dc_bios create")
Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>