COMPMID-3930: Update CLGEMM heuristic for fp16. Mali-G76

COMPMID-3930: Update CLGEMM heuristic for fp16. Mali-G76

- Since the GEMM kernel can now work without padding, the heuristic
  requires to be fine-tuned to exploit this feature
- The heuristic affects Mali-G76 FP16 only

Change-Id: Ia430627f02131ad956ce2219b80c83c8e7cabaf2
Signed-off-by: Gian Marco Iodice <gianmarco.iodice@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/4284
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Michele Di Giorgio <michele.digiorgio@arm.com>
Reviewed-by: SiCong Li <sicong.li@arm.com>