drm/i915/selftest: Simplify Y-major tiling in blit selftest

Rather than picking random tiling formats from a pool that contains both
TileY and Tile4 and then trying to replace one with the other depending
on the platform, it's simpler to just use a single enum value that
represents whatever the platform-appropriate Y-major tiling format is
(i.e., Tile4 on Xe_HP and beyond, legacy TileY on earlier platforms).

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Haridhar Kalvala <haridhar.kalvala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230810234618.3738870-3-matthew.d.roper@intel.com