drm/vmwgfx: validate the screen formats

The kms code wasn't validating the modifiers and was letting through
unsupported formats. rgb8 was never properly supported and has no
matching svga screen target format so remove it.
This fixes format/modifier failures in kms_addfb_basic from IGT.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220318174332.440068-4-zack@kde.org