drm/amd/display: Remove unused function bios_get_vga_enabled_displays

bios_get_vga_enabled_displays has been unused since
  commit 5a8132b9 ("drm/amd/display: remove dead dc vbios code")

Remove it.

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