Is the Fajr prayer valid, or must it be repeated after seeing spots on the trousers upon waking up and performing ablution to offer the prayer?

Tampering with the private parts is bad conduct, and if it is for lust, it is forbidden. Therefore, one must abstain from this practice and repent to Allah. What you did by washing the mentioned drops and performing ablution afterwards is correct, because it appears that what came out was madhy (pre-seminal fluid), and for its purification, washing the body and clothes it touched is sufficient according to the majority of scholars. If you are unsure whether the drops are maniy (seminal fluid) or madhy, please refer to Fatwa no. 113301.