Intel framebuffer compression & tiling update

[Jesse Barnes]

We've made some recent changes in the Intel git tree, enabling both 
tiled rendering for the front buffer by default, and framebuffer 
compression by default where available (currently, framebuffer 
compression is only available on mobile devices, e.g. 915GM, 965GM).

Tiled frontbuffer rendering should bring us some modest performance 
increases for certain operations (render acceleration specifically) 
though I haven't done any measurements on it.

Framebuffer compression is an Intel feature that uses run length 
encoding (RLE) to compress the front buffer to a separate compressed 
buffer from which actual scanout occurs.  When enabled, if your desktop 
is amenable to RLE, it can save as much as 0.7W in configurations I've 
tested, due to reduced memory bandwidth consumption during scanout 
(i.e. reading a compressed scanout buffer uses less bw than reading the 
full, uncompressed one).  It shouldn't measurably affect performance 
adversely, since we've configured it to re-compress the front buffer 
only every 1000 vblank events, nor should it increase power consumption 
in any measurable way, even in pathological configurations.

As usual, please report any bugs related to the above or any other Intel 
issue to freedesktop.org bugzilla, ideally after testing with the 
latest bits from git.

Thanks,
Jesse