drm/i915/bxt: Remove the preliminary_hw_support flag

Broxton is now part of CI which doesn't indicate any major problems so
enable the driver by default.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467384045-17028-1-git-send-email-imre.deak@intel.com