drm/i915: convert pointless error checks in set_config to BUGs

Because they all are, the ioctl command never calls us with any of
these violated. Also drop a equally pointless empty debug message (and
also in set_cursor, while we're at it).

With all these changes, intel_crtc_set_config is neatly condensed down
to it's essence, the actual modeset code (or fb update calling code)

v2: The fb helper code is actually stretching ->set_config semantics a bit,
it calls it with set->mode == NULL but set->fb != NULL.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>