drm/i915: Select CONFIG_SHMEM

The driver requires shmfs as the backing filesystem to handle the buffer
objects, so ensure it is selected if the user chooses to build our
driver.

Fixes: Bug 14662 - Dell E5500 kernel panic with KMS
http://bugzilla.kernel.org/show_bug.cgi?id=14662

The revealing nature of the panic is the NULL function pointer
dereference in read_cache_page_async().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reported-and-tested-by: Mateusz Kaduk <mateusz.kaduk@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Cc: stable@kernel.org