drm/amd/display: Add seamless boot support for more DIG operation modes

[Why]
When pre-OS firmware enables display support for displays that operate
the DIG in 2 pixels per cycle processing modes the inferred pixel rate
from get_pixel_clk_frequency_100hz does not account for the true pixel
rate since we're outputting 2 per cycle past the stream encoder.

This causes seamless boot validation to abort early.

[How]
Add a new stream encoder function for getting pixels per cycle from the
stream encoder. If the pixels per cycle is greater than 1 and the driver
policy is to enable 2 pixels per cycle for post-OS then allow seamless
boot to continue.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Duncan Ma <duncan.ma@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>