drm/amd/display: Check disable_fec flag before enabling fec.

drm/amd/display: Check disable_fec flag before enabling fec.

[Why]
dc debug option disable_fec was not working.

[How]
Check dc debug option disable_fec flag before
enabling fec in dp_should_enable_fec().

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>