drm/ttm/tests: Set DMA mask in KUnit device

drm/ttm/tests: Set DMA mask in KUnit device

In commit d393acce7b3f ("drm/tests: Switch to kunit devices"),
DRM test helpers migrated away from using a dummy platform driver
in favour of KUnit device. This means that DMA masks for the device
are not set but are required by ttm_pool_alloc tests.

Set the DMA mask for coherent mappings to unblock testing.

Signed-off-by: Karolina Stolarek <karolina.stolarek@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/dbd83b01bc835d922c070ad1c5bd9023e631e6c4.1718192625.git.karolina.stolarek@intel.com