Is it possible to implant bionic sub-retinal photoreceptors close to damaged retinal cells from trauma or diabetic retinopathy to make them function better?

Not Yet: That would be some very sophisticated technology and surgery to be able to do that. And as of yet nothing even comes close. Fda just approved the first retinal prosthesis for patients with rp and this is a big step toward what you are asking.

Maybe: Currently the argus ii is fda approved for use in eyes for a stringent subset of retinitis pigmentosa patients however, they have looked at implants for use in patients with other photoreceptor diseases such as macular degeneration. Who knows if the indications will extend to other things. There is a group on germany who is looking at subretinal implants. Argus ii is a epiretinal implant.

No improvement: Damaged photoreceptive cells will not function and therefore cannot send a signal up the optic nerve to register vision Bionic implants current in development have a light receptor array and a series of contact pins underneath to stimulate the remaining nerves. This creates a pattern of light and dark which the patient can interpret. The receptor cells are irrelevant.