drm/i915: Add support for drm syncobjs

This commit adds support for waiting on or signaling DRM syncobjs as
part of execbuf.  It does so by hijacking the currently unused cliprects
pointer to instead point to an array of i915_gem_exec_fence structs
which containe a DRM syncobj and a flags parameter which specifies
whether to wait on it or to signal it.  This implementation
theoretically allows for both flags to be set in which case it waits on
the dma_fence that was in the syncobj and then immediately replaces it
with the dma_fence from the current execbuf.

v2:
 - Rebase on new syncobj API
v3:
 - Pull everything out into helpers
 - Do all allocation in gem_execbuffer2
 - Pack the flags in the bottom 2 bits of the drm_syncobj*
v4:
 - Prevent a potential race on syncobj->fence

Testcase: igt/gem_exec_fence/syncobj*
Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1499289202-25441-1-git-send-email-jason.ekstrand@intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170815145733.4562-1-chris@chris-wilson.co.uk