drm/i915/gen11: Enable 6 sprites on gen11

Gen11 supports 7 planes + 1 cursor on each pipe. Bump
I915_MAX_PLANES to 8, and set num_sprites correctly.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[mlankhorst: Move the skl/bxt comment to the BXT branch. (Matt)]
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181018115134.9061-3-maarten.lankhorst@linux.intel.com