White patches inside both inner cheeks dried mud apperance dentist and doctor say cheek bitting frictional keratosis is this apperance normal.

Smoker?? If you are a heavy smoker than you have cancer risk, otherwise trust your doctors answer. If you are still worried, than request a biopsy.
Yes. Yes. The rubbing causes the tissue of your cheek to get thicker and blocks out the normal color of your cheek giving you the whitish brown color you are describing. You probably could use a night guard to help.