drm/i915: Remove dma_resv_prune

The signaled bit is already used for quick testing if a fence is signaled.
On top of that, it's a terrible abuse of dma-fence api, and in the common
case where the object is already locked by the caller, the trylock will fail.

If it were useful, the core dma-api would have exposed the same functionality.

The fact that i915 has a dma_resv_utils.c file should be a warning that the
functionality either belongs in core, or is not very useful at all.
In this case the latter.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[mlankhorst: Improve commit message]
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211021103605.735002-3-maarten.lankhorst@linux.intel.com
Reviewed-by: Matthew Auld <matthew.auld@intel.com> #irc