drm/amdgpu: hold a runtime PM reference for P2P dma-buf attachments

amdgpu_dma_buf_map() adds VRAM to the allowed domains for a peer2peer
attachment.  GTT is only a fallback placement when VRAM is preferred, so
ttm_bo_validate() migrates the buffer from GTT into VRAM.  While the
exporting device is runtime suspended its SDMA rings are down and the
move fails:

  amdgpu: Move buffer fallback to memcpy unavailable

An importer on a second GPU reaches this holding no runtime PM
reference on the exporter, e.g. a compositor on the APU submitting a
frame that references a buffer exported by an idle dGPU:

  amdgpu_cs_ioctl -> amdgpu_cs_parser_bos -> amdgpu_cs_bo_validate
    -> ttm_bo_validate -> amdgpu_bo_move -> dma_buf_map_attachment
      -> amdgpu_dma_buf_map -> ttm_bo_validate -> amdgpu_bo_move

Pinning a dma-buf into VRAM has the same requirement, which
commit 030631e97b20 ("drm/amdgpu: revert "take runtime pm reference
when we attach a buffer" v2") called out as the one case that would
need the reference back.

Take it in attach and drop it in detach.  pm_runtime_get_if_active()
never resumes the device, so it cannot deadlock against the reservation
taken during resume, which is why the old pm_runtime_get_sync() had to
go.  If the device is not active, clear peer2peer instead: the buffer
then stays in GTT, which remains accessible while the GPU is powered
down.  If runtime PM is disabled, take a plain reference so the put in
detach stays balanced.

Fixes: 030631e97b20 ("drm/amdgpu: revert "take runtime pm reference when we attach a buffer" v2")
Suggested-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Mike Lothian <mike@fireburn.co.uk>
Assisted-by: Claude:Opus-5 [Claude Code]
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 062ff15e30a48d14fb7d7558eba84f8dc97197f0)
Cc: stable@vger.kernel.org