drm/amd/display: Remove redundant if conditions

[WHY]
Coverity analysis flagged the two if conditions in
dcn30_dpp.c and dcn401_dpp.c as DEADCODE since they
are never true, since the variable
'program_prealpha_dealpha' is initialized at 0 and
never chagnes.

[HOW]
Removed the variable 'program_prealpha_dealpha' and
the if conditions.

Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Ivan Lipski <ivlipski@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>