drm/amd/display: Remove unused variables from vba_vars_st

Some variables from the struct vba_vars_st are not referenced in any
other place on the codebase. As they are not used, this commit removes
those variables.

Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>