kfdtest: decrease granularityMB for handling small vram system

It's not possible to allocate the 3/4 vram size with granularityMB
being 128 when vram size < 512MB and decrease granularityMB to 16 has
no significant impact on ROCt test on other system. So let's decrease
granularityMB on small vram system for handling LargestVramBufferTest().

Change-Id: Iea7c29abfd382a20761b653730fd09a220ad2fd0
Signed-off-by: Prike Liang <Prike.Liang@amd.com>

tests/kfdtest/src/KFDMemoryTest.cpp

@@ -965,7 +965,7 @@ void KFDMemoryTest::BinarySearchLargestBuffer(int allocNode, const HsaMemFlags &
     int ret;
 
     HsaMemMapFlags mapFlags = {0};
-    HSAuint64 granularityMB = 128;
+    HSAuint64 granularityMB = highMB > 512 ? 128 : 16;
 
     /* Testing big buffers in VRAM */
     unsigned int * pDb = NULL;