vulkan: use fp32 in coopmat2 q4_k dequant function (ggml-org#12309)

Author: jeffbolznv

ggml/src/ggml-vulkan/vulkan-shaders/dequant_funcs_cm2.comp

@@ -178,7 +178,7 @@ float16_t dequantFuncQ4_K(const in decodeBufQ4_K bl, const in uint blockCoords[2
 
     uvec4 v = bl128.block.q4k[0];
 
-    const f16vec2 loadd = unpackFloat2x16(v.x);
+    const vec2 loadd = vec2(unpackFloat2x16(v.x));
 
     uint32_t sc;
     uint32_t mbyte;
@@ -199,15 +199,15 @@ float16_t dequantFuncQ4_K(const in decodeBufQ4_K bl, const in uint blockCoords[2
     sc &= 0x3F;
     mbyte &= 0x3F;
 
-    const float16_t d = loadd.x * float16_t(sc);
-    const float16_t m = loadd.y * float16_t(mbyte);
+    const float d = loadd.x * float(sc);
+    const float m = loadd.y * float(mbyte);
 
     uint qs = uint32_t(bl16.block.qs[((idx & 0xC0) >> 2) + ((idx & 0x1E) >> 1)]);
     qs = (qs >> (b * 4 + 8 * (idx & 1))) & 0xF;
 
-    float16_t ret = d * float16_t(qs) - m;
+    float ret = d * float(qs) - m;
 
-    return ret;
+    return float16_t(ret);
 }
 
 layout(buffer_reference, std430, buffer_reference_align = 16) buffer decodeBufQ5_K {