i915: Add a Kconfig option to turn on i915.preliminary_hw_support by default

When building kernels for a preliminary hardware target, having to add a
kernel command-line option can prove inconvenient.  Add a Kconfig option
that changes the default of this option to 1.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Pimp the Kconfig help text a bit as suggested by Damien in
his 2nd review.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>