drm/amd/display: correct check of coverage blend mode

Check the value of per_pixel_alpha to decide whether the Coverage pixel
blend mode is applicable or not.

Fixes: 76818cdd ("drm/amd/display: add Coverage blend mode for overlay plane")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>