[Why]
Driver is restricting voltage levels if system cannot switch
in vblank.

[How]
Change allow_for_pstate_or_stutter_in_vblank_final from
dm_prefetch_support_uclk_fclk_and_stutter to
dm_prefetch_support_uclk_fclk_and_stutter_if_possible.
Add support for a new registry property,
DalDMLDisallowAlternatePrefetchModes, for easier debugging.
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: David Galiffi <David.Galiffi@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>