drm/amd/display: advertise PIXEL_NONE and PREMULTI blend mode for DCE

DCE can support PREMULTI and COVERAGE blend mode depending on its
generation, however current driver implementation either doesn't expose
more than primary and cursor plane, or doesn't program registers for any
blend mode other than PIXEL_NONE. To fix the missing-blend-mode-prop
warning according to current DCE plane caps, create blend mode property
with PIXEL_NONE and PREMULTI for primary planes. As long as the
background is black and there is no overlay plane, PIXEL_NONE and
PREMULTI are equivalent, and PREMULTI has been the mandatory/default
mode for years, so keep it to avoid regressions.

Fixes: 860e748bddcc ("drm: ensure blend mode supported if pixel format with alpha exposed")
Tested-by: Viktor Jägersküpper <viktor_jaegerskuepper@freenet.de>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com> #v3
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Reviewed-by: Leandro Ribeiro <leandro.ribeiro@collabora.com> #v2
Signed-off-by: Melissa Wen <mwen@igalia.com>
Link: https://patch.msgid.link/20260826104143.39077-4-mwen@igalia.com

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