Fix pointer tests in error-handling paths

In the error-handling paths of i915_gem_do_execbuffer() and
intel_crtc_page_flip(), the local pointer-to-request variables
were expected to be either valid pointers or NULL. Since

  26827088

 drm/i915: simplify allocation of driver-internal requests

they could also be ERR_PTR() values, so the tests need to be
updated to accommodate this case.

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453978089-29127-1-git-send-email-tvrtko.ursulin@linux.intel.com