Fix pixmap cache corruption when Xv was in use due to a memory allocator

calculation problem.

src/i830_video.c

@@ -1430,13 +1430,9 @@ I830AllocateMemory(ScrnInfoPtr pScrn, FBLinearPtr linear, int size)
 {
    ScreenPtr pScreen;
    FBLinearPtr new_linear;
-   int bytespp = pScrn->bitsPerPixel >> 3;
 
    DPRINTF(PFX, "I830AllocateMemory\n");
 
-   /* convert size in bytes into number of pixels */
-   size = (size + bytespp - 1) / bytespp;
-
    if (linear) {
       if (linear->size >= size)
 	 return linear;