drm/msm: Remove uneeded platform dev members

Commit eeb75474 ("drm/msm/gpu: use pm-runtime") adds a pointer
for the GPU platform device to the msm_gpu struct so we can
happily remove the same pointers from the individual GPU
structs.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>