drm/amd/display: add Coverage blend mode for overlay plane

According to the KMS man page, there is a
"Coverage" alpha blend mode that assumes the
pixel color values have NOT been pre-multiplied
and will be done when the actual blending to
the background color values happens.

Previously, this mode hasn't been enabled
in our driver and it was assumed that all
normal overlay planes are pre-multiplied
by default.

When a 3rd party app is used to input a image
in a specific format, e.g. PNG, as a source
of a overlay plane to blend with the background
primary plane, the pixel color values are not
pre-multiplied. So by adding "Coverage" blend
mode, our driver will support those cases.

Issue fixed: Overlay plane alpha channel blending is incorrect
Issue tracker: drm/amd#1769

Reference:
https://dri.freedesktop.org/docs/drm/gpu/drm-kms.html#plane-composition-properties



Adding Coverage support also enables IGT
kms_plane_alpha_blend Coverage subtests:
1. coverage-7efc
2. coverage-vs-premult-vs-constant

Changes
1. Add DRM_MODE_BLEND_COVERAGE blend mode capability
2. Add "pre_multiplied_alpha" flag for Coverage case
3. Read the correct flag and set the DCN MPCC
pre_multiplied register bit (only on overlay plane)

Signed-off-by: Sung Joon Kim <Sungjoon.Kim@amd.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Change-Id: I615270a32ec796f5d0080fb79fc14befd515d081