gpu: nvgpu: WAR for bad GPFIFO entries from userspace

gpu: nvgpu: WAR for bad GPFIFO entries from userspace

Userspace sometimes sends GPFIFO entries with a zero length. This
is a problem when the address of the pushbuffer of zero length is
larger than 32 bits. The high bits are interpreted as an opcode and
either triggers an operation that should not happen or is trated as
invalid.

Oddly, this WAR is only necessary on simulation.

Change-Id: I8be007c50f46d3e35c6a0e8512be88a8e68ee739
Signed-off-by: Alex Waterman <alexw@nvidia.com>
Reviewed-on: http://git-master/r/753379
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Tested-by: Terje Bergstrom <tbergstrom@nvidia.com>