drm/i915: Enable atomic support by default on supported platforms.

i915 is pretty much feature complete. Support for atomic i915-specific
connector properties is still missing; those properties can (for now)
only be set through the legacy ioctl.

ILK style watermarks and gen9+ watermarks are handled atomically,
and nonblocking modesets work. FBC has also been made to work with
atomic.

gen4x- and vlv/chv watermarks still need to be fixed, so disable atomic
by default there for now.

Flip the switch!!

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Lyude <cpaul@redhat.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1486021302-24910-1-git-send-email-maarten.lankhorst@linux.intel.com


[mlankhorst: Fix checkpatch warning about extra space in match_info]
Acked-by: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Lyude <lyude@redhat.com>