drm/amd/display: Attempt to avoid empty TUs when endpoint is DPIA

commit c03d770c

 upstream.

[WHY]
Empty SST TUs are illegal to transmit over a USB4 DP tunnel.
Current policy is to configure stream encoder to pack 2 pixels per pclk
even when ODM combine is not in use, allowing seamless dynamic ODM
reconfiguration. However, in extreme edge cases where average pixel
count per TU is less than 2, this can lead to unexpected empty TU
generation during compliance testing. For example, VIC 1 with a 1xHBR3
link configuration will average 1.98 pix/TU.

[HOW]
Calculate average pixel count per TU, and block 2 pixels per clock if
endpoint is a DPIA tunnel and pixel clock is low enough that we will
never require 2:1 ODM combine.

Cc: stable@vger.kernel.org # 6.6+
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>