i965/msaa: Add an enum to describe MSAA layout.

i965/msaa: Add an enum to describe MSAA layout.

From the Ivy Bridge PRM, Vol 1 Part 1, p112:

    There are three types of multisampled surface layouts designated
    as follows:
      - IMS Interleaved Multisampled Surface
      - CMS Compressed Mulitsampled Surface
      - UMS Uncompressed Multisampled Surface

Previously, the i965 driver only used IMS and UMS formats, and
distinguished beetween them using the boolean
intel_mipmap_tree::msaa_is_interleaved.  To facilitate adding support
for the CMS format, this patch replaces that boolean (and other
booleans derived from it) with an enum
INTEL_MSAA_LAYOUT_{IMS,CMS,UMS}.  It also updates the terminology used
in comments throughout the driver to match the IMS/CMS/UMS terminology
used in the PRM.  CMS layout is not yet used.

The enum has a fourth possible value, INTEL_MSAA_LAYOUT_NONE, which is
used for non-multisampled surfaces.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>