drm/amdgpu: Move coredump code to amdgpu_reset file

Giving that we use codedump just for device resets, move it's functions
and structs to a more semantic file, the amdgpu_reset.{c, h}.

Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
Reviewed-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>