drm/msm: Handle in-place remaps

Detect and handle the special case of a MAP op simply updating the vma
flags of an existing vma, and skip the pgtable updates.  This allows
turnip to set the MSM_VMA_DUMP flag on an existing mapping without
requiring additional synchronization against commands running on the
GPU.

Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Tested-by: Connor Abbott <cwabbott0@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/667238/