Allow to disable Raspberry Pi fb_copyarea acceleration via xorg.conf

Now acceleration is only used in the case if the AccelMethod option
is not set (so that it is assumed to be a default choice) or when
it is explicitly set to "COPYAREA". Any other value (for example
"CPU") disables acceleration.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

src/fbdev.c

@@ -942,12 +942,23 @@ FBDevScreenInit(SCREEN_INIT_ARGS_DECL)
 	}
 
 	if (!fPtr->SunxiG2D_private && fPtr->fb_copyarea_private) {
+		if (!(accelmethod = xf86GetOptValString(fPtr->Options, OPTION_ACCELMETHOD)) ||
+						strcasecmp(accelmethod, "copyarea") == 0) {
 			fb_copyarea_t *fb = fPtr->fb_copyarea_private;
 			if ((fPtr->SunxiG2D_private = SunxiG2D_Init(pScreen, &fb->blt2d))) {
 				fb->fallback_blt2d = &cpu_backend->blt2d;
 				xf86DrvMsg(pScrn->scrnIndex, X_INFO,
 				           "enabled fbdev copyarea acceleration\n");
 			}
+			else {
+				xf86DrvMsg(pScrn->scrnIndex, X_INFO,
+				           "failed to enable fbdev copyarea acceleration\n");
+			}
+		}
+		else {
+			xf86DrvMsg(pScrn->scrnIndex, X_INFO,
+			           "fbdev copyarea acceleration is disabled via AccelMethod option\n");
+		}
 	}
 
 	if (!fPtr->SunxiG2D_private && cpu_backend->cpuinfo->has_arm_vfp) {