drm/i916: add "auto" pipe CRC source

On gmch platforms the normal pipe source CRC registers don't work for
DP and TV encoders. And on newer platforms the single pipe CRC has
been replaced by a set of CRC at different stages in the platform.

Now most of our userspace tests don't care one bit about the exact
CRC, they simply want something that reflects any changes on the
screen. Hence add a new auto target for platform agnostic tests to
use.

v2: Pass back the adjusted source so that it can be shown in debugfs.

v3: I seem to be unable to get a stable CRC for DP ports. So let's
just disable them for now when using the auto mode. Note that
testcases need to be restructured so that they can dynamically skip
connectors. They also first need to set up the desired mode
configuration, since otherwise the auto mode won't do the right thing.

v4: Don't leak the modeset mutex on error paths.

v5: Spelling fix for the i9xx auto_source function.

Cc: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>