drm/i915: Rename for_each_plane -> for_each_universal_plane

This macro's name is a bit misleading; it doesn't actually iterate over
all planes since it omits the cursor plane.  Its only uses are in gen9
code which is using it to iterate over the universal planes (which we
treat as primary+sprites); in these cases the legacy cursor registers
are programmed independently if necessary.  The macro's iterator value
(0 for primary plane, spritenum+1 for each secondary plane) also isn't
meaningful outside the gen9 context where the hardware considers them to
all be "universal" planes that follow this numbering.

This is just a renaming/clarification patch with no functional change.
However it will make the subsequent patches more clear.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477522291-10874-2-git-send-email-matthew.d.roper@intel.com
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>