linux-core Makefile: add GIT_REVISION

This tries to automatically fetch a git revision string and if succeeds,
it #defines GIT_REVISION string macro. Packagers can override it by
'make GIT_REVISION=foo'.

Update Nouveau to use GIT_REVISION, if defined, instead of DRIVER_DATE
in struct drm_driver.

Signed-off-by: Pekka Paalanen <pq@iki.fi>