drm/amd/display: Underflow workaround by increasing SR exit latency

[Why]
On 14us for exit latency time causes underflow for 8K monitor with HDR on.
Increasing the latency to 28us fixes the underflow.

[How]
Increase the latency to 28us. This workaround should be sufficient
before we figure out why SR exit so long.

Reviewed-by: Chaitanya Dhere <chaitanya.dhere@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Nicholas Susanto <nicholas.susanto@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>