virgl: limit command length to 16 bits

virgl: limit command length to 16 bits

Much of our logic is based around the idea the upper 16 bits
of a command dword can encode the length of the command.

Now that the command buffer >= 2^16 - 1, we should check for
this.

v2: alignment, and only check VIRGL_ENCODE_MAX_DWORDS
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>