drm/amdgpu: GPU vs CPU page size fixes in amdgpu_vm_bo_split_mapping

start / last / max_entries are numbers of GPU pages, pfn / count are
numbers of CPU pages. Convert between them accordingly.

Fixes badness on systems with > 4K page size.

Cc: stable@vger.kernel.org
Bugzilla: https://bugs.freedesktop.org/106258
Reported-by: Matt Corallo <freedesktop@bluematt.me>
Tested-by:  <foxbat@ruin.net>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>