drm/fourcc: 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.

Fixes: 8ba16d59 ("drm/fourcc: Add AMD DRM modifiers.")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>