drm/amd/display: ignore modifiers when checking for format support

[Why&How]
There are cases where swizzle modes are set but modifiers arent. For
such a userspace, we need not check modifiers while checking
compatibilty in the drm hook for checking plane format.

Ignore checking modifiers but check the DCN generation for the
supported swizzle mode.

v2: squash in unused variable removal (Alex)

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>