drm/i915: Move num_plane to the intel_device_info structure

And rename it to num_sprites as this value doesn't count the primary
plane.

This limit lives with num_pipes really, and now that dev_priv->info is
writable we can put it there instead.

While at it, introduce a intel_device_info_runtime_init() where we'll be
able to gather the device info fields at run-time.

v2: rename num_plane to num_sprites (Ville Syrjälä)
v3: rebase on top of latest drm-nightly

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com> (for v2)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (for v2)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>