drm/i915: Add request to execbuf params and add explicit cleanup

Rather than just having a local request variable in the execbuff code, the
request pointer is now stored in the execbuff params structure. Also added
explicit cleanup of the request (plus wiping the OLR to match) in the error
case. This means that the execbuff code is no longer dependent upon the OLR
keeping track of the request so as to not leak it when things do go wrong. Note
that in the success case, the i915_add_request() at the end of the submission
function will tidy up the request and clear the OLR.

For: VIZ-5115
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Tomas Elf <tomas.elf@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>