drm/amdkfd: Fix variable dereferenced before NULL check in 'kfd_dbg_trap_device_snapshot()'

Fixes the below:
drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_debug.c:1024 kfd_dbg_trap_device_snapshot() warn: variable dereferenced before check 'entry_size' (see line 1021)

Cc: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>