msm: kgsl: Remove unused functions from gmu_core_ops

get_idle_level() and set_idle_level() not implemented
for any of the gmu. Hence removing these functions.

Change-Id: Ifc3fa216fb8f9fa298aba00dc6af4040f107f5fe
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>