vpu5: Prevent register re-initialization while decoder is running

This rearranges the following initialization procedures to assure
that they are performed only once even if multiple instances are initiated
simultaneously.

- Intializing the VPC.
- Resetting the VPU's interrupt registers.
- Clearing variables for saving interrupt mask bits.
vpu5: deinit: Fix using one counter for two loops

The same counter was mistakenly used for both the outer and inner loops
when freeing working buffers on component deinitialization.