fix overflow warning on videoRam

Note that pScrn->videoRam is an 'int'.

i830_driver.c: In function ‘I830ScreenInit’:
i830_driver.c:3019: warning: overflow in implicit constant conversion

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

src/i830_driver.c

@@ -3016,7 +3016,7 @@ I830ScreenInit(int scrnIndex, ScreenPtr pScreen, int argc, char **argv)
    if (pI830->use_drm_mode) {
 #ifdef XF86DRM_MODE
        pI830->stolen_size = 0;
-       pScrn->videoRam = ~0UL / KB(1);
+       pScrn->videoRam = INT_MAX / KB(1);
 #endif
    } else {
        I830AdjustMemory(pScreen);