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: 76818cd ("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>

Author: melissawen

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -5446,7 +5446,7 @@ fill_blending_from_plane_state(const struct drm_plane_state *plane_state,
 			}
 		}
 
-		if (per_pixel_alpha && plane_state->pixel_blend_mode == DRM_MODE_BLEND_COVERAGE)
+		if (*per_pixel_alpha && plane_state->pixel_blend_mode == DRM_MODE_BLEND_COVERAGE)
 			*pre_multiplied_alpha = false;
 	}