drm/amdgpu: remove the adev check for NULL

adev is a global data structure and isn't expected
to be NULL and hence removing the redundant adev
check from the devcoredump code.

Cc: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>