drm/i915: Don't populate pipe_src_{w,h} multiple times

If we ever end up doing the retry loop due to bandwidth constraints, we
would rewrite pipe_src_{w,n} based on adjusted_mode timings. But by that
time the encoder may have already replaced the adjusted_mode with a
fixed panel mode, which would then corrupt pipe_src_{w,h}.

v2: Use requested_mode and slap on a big comment from Daniel

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>