drm/vmwgfx: limit screen size to stdu_max during check_modeset

For STDU individual screen target size is limited by
SVGA_REG_SCREENTARGET_MAX_WIDTH/HEIGHT registers so add that limit
during atomic check_modeset.

An additional limit is placed in the update_layout ioctl to avoid
requesting layouts that current user-space typically can't support.
Also modified the comments to reflect current limitation on topology.

Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>