drm/amd/display: don't ignore alpha property on pre-multiplied mode

drm/amd/display: don't ignore alpha property on pre-multiplied mode

"Pre-multiplied" is the default pixel blend mode for KMS/DRM, as
documented in supported_modes of drm_plane_create_blend_mode_property():
https://cgit.freedesktop.org/drm/drm-misc/tree/drivers/gpu/drm/drm_blend.c

In this mode, both 'pixel alpha' and 'plane alpha' participate in the
calculation, as described by the pixel blend mode formula in KMS/DRM
documentation:

out.rgb = plane_alpha * fg.rgb +
          (1 - (plane_alpha * fg.alpha)) * bg.rgb

Considering the blend config mechanisms we have in the driver so far,
the alpha mode that better fits this blend mode is the
_PER_PIXEL_ALPHA_COMBINED_GLOBAL_GAIN, where the value for global_gain
is the plane alpha (global_alpha).

With this change, alpha property stops to be ignored. It also addresses
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1734

v2:
 * keep the 8-bit value for global_alpha_value (Nicholas)
 * correct the logical ordering for combined global gain (Nicholas)
 * apply to dcn10 too (Nicholas)

Signed-off-by: Melissa Wen <mwen@igalia.com>
Tested-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Tested-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>