Too many scanlines for pixmap cache on i810

[Ken Mandelberg]

The latest git version of the modset driver seems to be miscomputing the 
number of scanlines for the pixmap cache on my 945GM.

It comes up with

(II) I810(0): Allocating at least 4344 scanlines for pixmap cache
(II) I810(0): Initial framebuffer allocation size: 21472 kByte
(EE) I810(0): Failed to allocate framebuffer. Is your VideoRAM set too low?

The driver computation is

          size = 3 * lineSize * pScrn->virtualY;
          size += 1920 * 1088 * 2 * 2;
          size = ROUND_TO_PAGE(size);

          cacheLines = (size + lineSize - 1) / lineSize;


lineSize is 1K*4, so cacheLines is roughly  3*virtualY +2K, and with 
virtualY at 768 that works out to about 4K.

Older versions of the driver gave

(II) I810(0): Allocating at least 256 scanlines for pixmap cache
(II) I810(0): Initial framebuffer allocation size: 7680 kByte
(II) I810(0): Allocating at least 256 scanlines for pixmap cache
(II) I810(0): Initial framebuffer allocation size: 7680 kByte