drm/i915: Lock timeline mutex directly in error path of eb_pin_timeline

Don't use the interruptable version of the timeline mutex lock in the
error path of eb_pin_timeline as the cleanup must always happen.

v2:
 (John Harrison)
  - Don't check for interrupt during mutex lock
v3:
 (Tvrtko)
  - A comment explaining why lock helper isn't used

Fixes: 544460c3 ("drm/i915: Multi-BB execbuf")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220111163929.14017-1-matthew.brost@intel.com