i965/dri: Handle Y-tiled modifier

i965/dri: Handle Y-tiled modifier

This patch begins introducing how we'll actually handle the potentially
many modifiers coming in from the API, how we'll store them, and the
structure in the code to support it.

Prior to this patch, the Y-tiled modifier would be entirely ignored. It
shouldn't actually be used until this point because we've not bumped the
DRIimage extension version (which is a requirement to use modifiers).

Measuring later in the series with kmscube:
Linear:
Read bandwidth: 1048.44 MiB/s
Write bandwidth: 1483.17 MiB/s

Y-tiled:
Read bandwidth: 471.13 MiB/s
Write bandwidth: 589.10 MiB/s

Similar functionality was introduced and then reverted here:

commit 6a0d036483caf87d43ebe2edd1905873446c9589
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Thu Apr 21 20:14:58 2016 -0700

    i965: Always use Y-tiled buffers on SKL+

v2: Use last set bit instead of first set bit in modifiers to address
bug found by Daniel Stone.

v3: Use the new priority modifier selection thing. This nullifies the
bug fixed by v2 also.

v4: Get rid of modifier compaction which originally served another
purpose and now serves none (Jason)

Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>