[AMDGPU] exp should not be in WQM mode

A mrt exp with vm=1 must be in exact (non-WQM) mode, as it also exports
the exec mask as the valid mask to determine which pixels to render.

This commit marks any exp as needing to be in exact mode.

Actually, if there are multiple mrt exps, only one needs to have vm=1,
and only that one needs to be in exact mode. But that is an optimization
for another day.

Differential Revision: https://reviews.llvm.org/D36305

llvm-svn: 312915