drm/amd/display: Delay PSR entry

[Why]
After panel power up, if PSR entry attempted too early,
PSR state may get stuck in transition.
This could happen if the panel is not ready
to respond to the SDP PSR entry message.
In this case dmub f/w is unable to abort PSR entry
since abortion is not permitted after the SDP has been sent.

[How]
Skip 5 pageflips before PSR enable.

Signed-off-by: Roman Li <roman.li@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>