drm/i915: Add i915.nuclear_pageflip command line param to force atomic (v4)

We don't have full atomic modeset support yet, but the "nuclear
pageflip" subset of functionality (i.e., plane operations only) should
be ready.  Allow the user to force atomic on for debug purposes, or for
fixed-purpose embedded devices that will only use atomic for plane
updates.

The term 'nuclear' is used here instead of 'atomic' to make it clear
that this doesn't allow full atomic modeset support, just a (very
useful) subset of the atomic functionality.

We'll drop the kernel parameter and unconditionally enable atomic in a
future patch once all of the necessary pieces are in.

v2:
 - Use module_param_named_unsafe() (Daniel)
 - Simplify comment on DRIVER_ATOMIC guard (Daniel)

v3:
 - Make the parameter "nuclear_pageflip" rather than just "nuclear"
   for clarity. (Ander)

v4:
 - Make the internal variable "nuclear_pageflip" as well as the
   command-line option. (Ander)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>