intel: initialize bufmgr.bo_mrb_exec unconditionally

If bufmgr.bo_mrb_exec is not set, drm_intel_bo_mrb_exec returns ENODEV
even though drm_intel_gem_bo_mrb_exec2 will work fine for the RENDER ring.
Fixes xf86-video-intel after commit 'add BLT ring support' (5bed685f76)
with kernels without BSD or BLT ring support (2.6.34 and before).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31443


Signed-off-by: Albert Damen <albrt@gmx.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
(cherry picked from commit 49447a9b)

Signed-off-by: Cyril Brulebois <kibi@debian.org>