amd/registers: fix fields conflict detection

amd/registers: fix fields conflict detection

The existing code handled the case where the new definition of the
same field was larger than the old one.
This commit adds a check to handle the reverse case: the new def
is smaller than the old one (= so writing using the merged macro
would affect the next fields).

The affected fields are:
* LGKM_CNT (in SQ_WAVE_IB_STS)
* DONUT_SPLIT (in VGT_TESS_DISTRIBUTION)
* HEAD_QUEUE (in GDS_GWS_RESOURCE)

DONUT_SPLIT is the only one used by radeonsi/radv.

Fixes: e6184b08924 ("amd/registers: scripts for processing register descriptions in JSON")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12063>