drm/doc: Remove outdated note about i915 driver not behaving properly

The i915 driver has been fixed not to modify the mode argument of the
encoder mode_fixup operation. Remove the related comment from the
documentation.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>