i965: Fix buffer overruns in MSAA MCS buffer clearing.

This manifested as rendering failures or sometimes GPU hangs in
compositors when they accidentally got MSAA visuals due to a bug in the X
Server.  Today we decided that the problem in compositors was equivalent
to a corruption bug we'd noticed recently in resizing MSAA-visual
glxgears, and debugging got a lot easier.

When we allocate our MCS MT, libdrm takes the size we request, aligns it
to Y tile size (blowing it up from 300x300=900000 bytes to 384*320=122880
bytes, 30 pages), then puts it into a power-of-two-sized BO (131072 bytes,
32 pages).  Because it's Y tiled, we attach a 384-byte-stride fence to it.
When we memset by the BO size in Mesa, between bytes 122880 and 131072 the
data gets stored to the first 20 or so scanlines of each of the 3 tiled
pages in that row, even though only 2 of those pages were allocated by
libdrm.  In the glxgears case, the missing 3rd page happened to
consistently be the static VBO that got mapped right after the first MCS
allocation, so corruption only appeared once window resize made us throw
out the old MCS and then allocate the same BO to back the new MCS.

Instead, just memset the amount of data we actually asked libdrm to
allocate for, which will be smaller (more efficient) and not overrun.
Thanks go to Kenneth for doing most of the hard debugging to eliminate a
lot of the search space for the bug.

Cc: "10.0 10.1" <[email protected]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77207
Reviewed-by: Kenneth Graunke <[email protected]>

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