Fixed _gemm_small() prototype; disabled gemm_small.

Details:
- Fixed a mismatch between the prototype for bli_gemm_small() in
  bli_gemm_front.h and the actual definition of bli_gemm_small() in
  kernels/zen/3/bli_gemm_small.c. The former was erroneously declaring
  the cntl_t* argument as 'const'. Thanks to Jeff Diamond for reporting
  this issue.
- Commented out BLIS_ENABLE_SMALL_MATRIX, BLIS_ENABLE_SMALL_MATRIX_TRSM
  macro definitions in config/zen3/bli_family_zen3.h. AMD's small matrix
  implementation should probably remain disabled in vanilla BLIS, at
  least for now.

Author: fgvanzee

config/zen3/bli_family_zen3.h

@@ -52,8 +52,8 @@
 // All zen3 specific code should be included in this macro
 #define BLIS_CONFIG_ZEN3
 
-#define BLIS_ENABLE_SMALL_MATRIX
-#define BLIS_ENABLE_SMALL_MATRIX_TRSM
+//#define BLIS_ENABLE_SMALL_MATRIX
+//#define BLIS_ENABLE_SMALL_MATRIX_TRSM
 
 
 // This will select the threshold below which small matrix code will be called.

frame/3/gemm/bli_gemm_front.h

@@ -52,7 +52,7 @@ err_t bli_gemm_small
        const obj_t*  beta,
        const obj_t*  c,
        const cntx_t* cntx,
-       const cntl_t* cntl
+             cntl_t* cntl
      );
 #endif