Find the gpu platform with the most devices and use that if no platform option is passed.

ocl.c

@@ -33,7 +33,7 @@
 #include "findnonce.h"
 #include "ocl.h"
 
-int opt_platform_id;
+int opt_platform_id = -1;
 
 char *file_contents(const char *filename, int *length)
 {
@@ -80,7 +80,7 @@ int clDevicesNum(void) {
 	cl_uint numPlatforms;
 	cl_platform_id *platforms;
 	cl_platform_id platform = NULL;
-	unsigned int most_devices = 0, i;
+	unsigned int most_devices = 0, i, mdplatform;
 
 	status = clGetPlatformIDs(0, NULL, &numPlatforms);
 	/* If this fails, assume no GPUs. */
@@ -123,8 +123,10 @@ int clDevicesNum(void) {
 			return -1;
 		}
 		applog(LOG_INFO, "Platform %d devices: %d", i, numDevices);
-		if (numDevices > most_devices)
+		if (numDevices > most_devices) {
 			most_devices = numDevices;
+			mdplatform = i;
+		}
 		if (numDevices) {
 			unsigned int j;
 			char pbuff[256];
@@ -139,6 +141,9 @@ int clDevicesNum(void) {
 		}
 	}
 
+	if (opt_platform_id < 0)
+		opt_platform_id = mdplatform;;
+
 	return most_devices;
 }