Workaround for partial huge page unmaps in Panthor

[Adrián Larumbe]

This is v4 of [1]. This patch is a workaround for performing partial unmaps of a VM region backed
by huge pages. Since these are now disallowed, the patch makes sure unmaps are done on a backing
page-granularity, and then regions untouched by the VM_BIND unmap operation are restored.

A patch series with IGT tests to validate this functionality is found at [2].

Changelog:
v5:
 - Addressed minor formating nit.
 - Passedd right range value to panthor_vm_lock_region() rather than end address.
 - Added R-b tag.

v4:
 - Added VM lock for expanded unmap region.
 - No longer pass the original VMA when calculating expanded unmap address and ranges for
 calculating BO offsets, as this can be obtained from the map va op's
 - Added more comments and renamed the unmap boundary calculation function
 - Calculate prev and next map offsets, sizes and addresses in the block prelude for
 the sake of clarity.
 - Addressed some minor style nits.
 - Rebased the patch onto the latest drm-misc.

v3:
 - Reworked address logic so that prev and next gpuava_op's va's are used in the calculations
   instead of those of the original unmap vma.
 - Got rid of the return struct from get_map_unmap_intervals() and now reckon panthor_vm_map_pages()
   arguments by fiddlign with the gpuva's respective gem object offsets.
 - Use folio_size() instead of folio_order() because the latter implies page sizes from the
   CPU's MMU perspective, rather than that of the GPU.

v2:
 - Fixed bug caused by confusion between semantics of gpu_va prev and next ops boundaries
   and those of the original vma object.
 - Coalesce all unmap operations into a single one.
 - Refactored and simplified code.

[1] https://lore.kernel.org/dri-devel/20251214202426.3186811-1-adrian.larumbe@xxxxxxxxxxxxx/
[2] https://lore.kernel.org/igt-dev/20251213190205.2435793-1-adrian.larumbe@xxxxxxxxxxxxx/T/#

Adrián Larumbe (1):
  drm/panthor: Support partial unmaps of huge pages

 drivers/gpu/drm/panthor/panthor_mmu.c | 100 +++++++++++++++++++++++---
 1 file changed, 92 insertions(+), 8 deletions(-)

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2.51.2