drm/amdkfd: Fix GCC 10 compiler warning

GCC 10 was complaining about how we append data to a buffer using snprintf:

drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_topology.c: In function ‘perf_show’:
drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_topology.c:214:3: warning: ‘snprintf’ argument 4 overlaps destination object ‘buf’ [-Wrestrict]
  214 |   snprintf(buffer, PAGE_SIZE, "%s"fmt, buffer, __VA_ARGS__)
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This patch fixes the warnings and makes the sysfs code more efficient
by remembering the offset in the buffer between append operations.
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Aaron Liu <aaron.liu@amd.com>
Tested-by: Aaron Liu <aaron.liu@amd.com>
Reviewed-by: Amber Lin <Amber.Lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>