drm/uapi: Fix modifier field mask for AMD modifiers.
The DCC_MAX_COMPRESSED_BLOCK has to contain one of AMD_FMT_MOD_DCC_BLOCK_* and with 3 values this doesn't fit in 1 bit.
Fix this cleanly while it is only in drm-next.
Found while reviewing Simon's drm_info PR: https://github.com/ascent12/drm_info/pull/63/commits/eaeae6ee78764a03d959cbc97c8b514f81a94c63#r523030172
Fixes: 2cc2b456 "drm-uapi: Add AMD modifiers."