drm-intel.rst: remove drm-intel-fixes@lists.freedesktop.org in favor Fixes:

drm-intel-fixes@lists.freedesktop.org is dead for a while already
and everybody is pretty much using 'Fixes:' tag properly. So it is
time to adjust the documentation to the real life. No change on
the flow though.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>