drm/i915: Don't set undefined bits in dirty_pipes

drm/i915: Don't set undefined bits in dirty_pipes

skl_commit_modeset_enables() straight up compares dirty_pipes
with a bitmask of already committed pipes. If we set bits in
dirty_pipes for non-existent pipes that comparison will never
work right. So let's limit ourselves to bits that exist.

And we'll do the same for the active_pipes_changed bitmask.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191011200949.7839-5-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>