intel/gma: Return success for Intel int15 VGA set panel type hook

One of the interrupts in intel_vga_int15_handler lacks
positive return status. Write correct status to avoid
error messages in log.

TEST=With this change `int15 call returned error` is not shown anymore
     on a custom board with Intel Atom CPU, i945GME northbridge and
     i82801gx southbridge.

Change-Id: I740b2df9bd6a7d261d89bef74b924edbb64354aa
Signed-off-by: Konstantin Aladyshev <aladyshev@nicevt.ru>
Reviewed-on: http://review.coreboot.org/12255
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/drivers/intel/gma/int15.c

@@ -60,6 +60,7 @@ int intel_vga_int15_handler(void)
 		X86_AX = 0x005f; // Success
 		X86_CL = panel_type;
 		printk(BIOS_DEBUG, "DISPLAY=%x\n", X86_CL);
+		res = 1;
 		break;
 	case 0x5f51:
 		/*