msm: kgsl: Properly take syncsource_lock spinlock

Syncsource spinlock is not properly taken while removing syncsource
id from the synsource_idr. idr_find() and idr_remove() should be called
atomically. Fix this by keeping idr_find() and idr_remove() inside
a single lock.

Change-Id: I51b8dcd0d67deda746c05e4cbd9eb1355d8f5d0b
Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>