drm/amd/display: Reset alpha state for planes to the correct values

[Why]
The plane_reset callback is subclassed but hasn't been updated since
the drm helper got updated to include resetting alpha related state
(state->alpha and state->pixel_blend_mode). The overlay planes
exposed by amdgpu_dm were therefore being rendered as invisible by
default ever since supported was exposed for alpha blending properties
on overlays.

This caused regressions in igt@kms_plane_multiple@atomic-tiling-none
and igt@kms_plane@plane-position-covered-pipe tests.

[How]
Reset the plane state values to their correct values as defined in
the drm helper.

This fixes the IGT test regression.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>