The vp9_denoiser_free() function will internally check if the
buffer pointers are NULL. This commit makes the encoder always
call vp9_denoiser_free() after finishing encoding. It protects the
case where noise_sensitivity_level is changed during encoding
process and happen to be turned off towards the end of sequence,
which could result memory space allocated to denoiser not being
released.

Change-Id: Ie20dc2f2e6e5fb6333fbab3356bc153978a6a0f8