drm/amd/display: Refactor dcn31_panel_construct to avoid assert

[Why]
We want to avoid unnecessary asserts, one of which is hit in
dcn31_panel_construct when booting on a DCN32 asic that has an eDP
connector on a different DIG than A or B. The DIG-based mapping only
applies when edp0_on_dp1 is supported, therefore the check for valid
eng_id can be moved within the appropriate section of the if statement.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>