drm/amdkfd: set coherent host access capability flag

This flag determines whether the host possesses coherent access to
the memory of the device.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>