msm: kgsl: Split out adreno_idle into a lighter weight function

In most cases idling the GPU is a heavy weight operation: stop the
dispatcher, wait for the hardware, deal with faults, etc. The idle
following ME_INIT is not one of those cases. The init sequence
is a controlled operation with a stopped dispatcher so we can forgo
the heavier bits of adreno_idle(). Break out the idle loop
and turn it into adreno_spin_idle() for use by both adreno_idle()
and ringbuffer init.

Change-Id: Ic0dedbad870ea65cc78d2a45d90ed895b87a7f5e
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>