blorp: Turn anv_CmdCopyBuffer into a blorp_buffer_copy() helper.

blorp: Turn anv_CmdCopyBuffer into a blorp_buffer_copy() helper.

I want to be able to copy between buffer objects using BLORP in the i965
driver.  Anvil already had code to do this, in a reasonably efficient
manner - first using large bpp copies, then smaller bpp copies.

This patch moves that logic into BLORP as blorp_buffer_copy(), so we
can use it in both drivers.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>