ati-vga: Implement foreground and background color register writes

These are straightforward 32-bit register write handlers. They're
necessary for a future patch which will use them for color expansion
from monochrome host data transfers.

Signed-off-by: Chad Jablonski <chad@jablonski.xyz>
Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-ID: <20260303024730.1489136-6-chad@jablonski.xyz>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

hw/display/ati.c

@@ -924,6 +924,12 @@ static void ati_mm_write(void *opaque, hwaddr addr,
     case DP_CNTL:
         s->regs.dp_cntl = data;
         break;
+    case DP_SRC_FRGD_CLR:
+        s->regs.dp_src_frgd_clr = data;
+        break;
+    case DP_SRC_BKGD_CLR:
+        s->regs.dp_src_bkgd_clr = data;
+        break;
     case DP_DATATYPE:
         s->regs.dp_datatype = data & 0xe0070f0f;
         break;