drm/vmwgfx: Get maximum mob size from register SVGA_REG_MOB_MAX_SIZE

This patch queries the register SVGA_REG_MOB_MAX_SIZE for the
maximum size of a single mob.

Signed-off-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>