panfrost: Compact the bo_access readers array

panfrost: Compact the bo_access readers array

Previously, the array bo_access->readers was only cleared when there
were no unsignaled fences, which in some situations never happened.

That resulted in the array having thousands of NULL pointers, but only
a handful of active readers.

With this patch, all the unsignaled readers are moved to the front of
the array, effectively building a new array only containing the active
readers in-place. This results in the readers array usually only having
a couple of elements.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3419>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3419>