drm/gpusvm: set has_dma_mapping inside mapping loop

The 'has_dma_mapping' flag should be set once there is a
mapping so it could be unmapped in case of error.

v2:
- Resend for CI

Fixes: 99624bdf ("drm/gpusvm: Add support for GPU Shared Virtual Memory")
Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250428024752.881292-1-matthew.brost@intel.com