gallium/dri2: Fix creation of multi-planar modifier images

The commit noted below assumed and enforced that DRM_MOD_INVALID was the
only valid modifier for multi-planar imported images. Due to that, it
required that modifier on multi-planar images to:

   1. Allow multiple planes.
   2. Perform YUV format lowering and extent adjustments.
   3. Use buffer_index to correctly map the given planes.

Fix these issues by removing or updating the code built on that
assumption.

Fixes: 2066966c ("gallium/dri2: Support creating multi-planar modifier images")
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit d5c85783)