panfrost: Track buffer initialization

panfrost: Track buffer initialization

We want to know if a given slice of a buffer is initialized at a
particular point in the execution of the program. This is accomplished
easily enough -- start out uninitialized and upon an operation writing
to the buffer, mark it initialized.

The motivation is to optimize away expensive operations (like wallpaper
blits) when reading from an uninitialized buffer; since it's
uninitialized, the results of these operations are undefined, and it's
legal to take the fast path ^_^

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>