i965/drm: Drop code to search for an existing bufmgr.

i965/drm: Drop code to search for an existing bufmgr.

This functionality was added by libdrm commit
743af59669386cb6e063fa4bd85f0a0b2da86295 (intel: make bufmgr_gem
shareable from different API) in an attempt to solve libva/mesa buffer
sharing problems.  Specifically, this was working around an issue hit
by Chromium, which used the same drm_fd for multiple APIs, and shared
buffers between them.

This code attempted to work around that issue by using the same bufmgr
for both libva and Mesa.  It worked because libdrm_intel was loaded by
both libraries.  However, now that Mesa has forked, we don't have a
common library, and this code cannot work.

The correct solution is to have each API open its own file descriptor
(and get a corresponding buffer manager), and then use PRIME export
and import to share BOs across those APIs.  Then the kernel can manage
those shared resources.  According to Chris, the kernel will pass back
the same handle for a prime FD if the lookup is from the same device FD.

We believe Chromium has since moved to this model.

In Mesa, there is already only one screen per FD, and so there will
only be one bufmgr per FD.  We don't need any of this code.

v2: Add a big warning comment written by Chris Wilson.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>