Remove i915 driver sources from linux-core

Intel developers have stated, that their DRM development continues
elsewhere in some Linux kernel trees. This makes the code in drm.git
just dead weight. This removal allows further cleanup of compatibility
code.

shared-core and bsd-core are left untouched this time.

Signed-off-by: Pekka Paalanen <pq@iki.fi>
Acked-by: Eric Anholt <eric@anholt.net>