msm: kgsl: Always submit an interrupt request to the GPU

A context does not have to be present to send an internal dummy
command to the GPU.  Remove BUG() in this case and instead send
a NULL context which will still cause the interrupt to get submitted
to the GPU.

CRs-fixed: 392688
Change-Id: I66b6eebe899ab09e6615d08faddea7ea73de5c6d
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>