More 16 vs 24 bpp profiling

[Stefano Fedrigo]

I've done some more profiling on the 16 vs. 24 bpp issue.
This time I used this test:
https://dev.laptop.org/git?p=sugar;a=blob;f=tests/graphics/hipposcalability.py

A simple speed test: I measured the time required to scroll down and up
one time all the generated list.  Not extremely accurate, but I repeated the
test a few times with consistent results (+- 0.5 secs).  Mean times:

xserver 1.4
16 bpp: 37.9
24 bpp: 40.7

xserver 1.3
16: 46.4
24: 50.1

At 24 bpp we're a little slower.  1.3 is 20% slower than 1.4. The pixman
migration patch makes the difference: 1.3 spend most of that 20% in memcpy().

The oprofile reports are from xserver 1.4.  I don't see much difference
between 16 and 24, except that at 24 bpp, less time is spent in pixman and more
in amd_drv.  At 16 bpp pixman_fill() takes twice the time.

Unfortunately without a working callgraph it's not very clear to me what's
happening in amd_drv.  At 24bpp gp_wait_until_idle() takes twice the time...

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