Well, as usual, that depends. If your display is somehow offensive or disruptive, you may legally be asked to take it down. If, for example, you were to put up a life size nativity scene that played Christmas carols all day, your employer would probably be justified in asking you to remove it. That said, employers have pretty broad discretion to control the workplace, and could prohibit all displays of a certain size, or nature.
However, employers cannot engage in or permit harassment of you on the basis of your religion. Depending on the circumstances, telling you to remove a religious display could be considered harassment, or allowing a hostile work environment based on religion. Further, your employer cannot take “adverse action” against you on the basis of your religion. That includes termination and discipline of other kinds.
A religious display could be considered, in some circumstances, a religious “accommodation” that your employer would have to consider. Whether it is required to provide the requested accommodation is evaluated based on the cost to the employer and the potential disruptive effect it could have on the workplace.
If you work for a government agency, the rules are different. The government cannot endorse any one particular religion, or give the appearance of endorsing any particular religion, and can prohibit employee religious displays that might give that impression. Government employers are also more tightly constrained by the First Amendment in their restriction on religious speech and action.