i965: Fix buffer object segfault since removal of system memory BOs.

i965: Fix buffer object segfault since removal of system memory BOs.

Commit cf31a19300cbcecddb6bd0f878abb9316ebad2a1 removed support for BOs
backed by system memory, as it was only useful for i915.  However, it
removed a little too much code: intel_bufferobj_buffer() used to call
intel_bufferobj_alloc_buffer(), and after that commit, it didn't.

This led to NULL pointer dereferences in several test cases, such as
es3conform's transform_feedback_state_variables test.

This commit restores the allocation, preserving the original behavior.
It may not be the cleanest approach, but tidying should come later.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66432
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>