[AMDGPU] Validate user SGPR count against HW range, not field width (#194449)
The previous validation checked only the field width, allowing values
that exceeded the actual hardware limits (e.g. 0–16 on gfx6-gfx120 and
0–32 on gfx125x) as long as they fit in the bit width.
Tighten validation to reject out-of-range user SGPR counts.