drm/amdkfd: Adjust CWSR trap handler for gfx950

In gfx950, the SQ_WAVE_LDS_ALLOC.LDS_SIZE field is extended to bits 12
to 22.  The LDS_SIZE granularity remains unchanged (units of 64 dwords,
or 256 bytes).  This patch adjusts the CWSR trap handler to read the
full extent of LDS_SIZE.

Signed-off-by: Lancelot SIX <lancelot.six@amd.com>
Reviewed-by: Jay Cornwall <jay.cornwall@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>