msm: kgsl: Get rid of gmu_core_snapshot and gmu_core_dev_snapshot

Assign the gmu specific snapshot function to gpudev->snapshot instead
of creating a gmu_core_snapshot. This function will send an nmi if we
hit a gmu fault. Then, it will capture gmu snapshot followed by the generic
a6xx_snapshot as against a6xx snapshot calling the gmu snapshot.
Do the same for rgmu. This helps get rid of gmu_core_snapshot and
gmu_core_dev_snapshot() functions.

Change-Id: I0f72a930633557260b59a76f0161bcde8c5a6da2
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>