[pvr] fix view mode not being set properly after channel switch under certain circumstances

This PR fixes the following two scenarios:

- previously the view mode was only changed during channel switch if it differed from the current mode. This meant that the view mode was not updated when switching between two channels that both use custom mode but have differing zoom amount.

- when switching from a channel with custom view mode and zoom amount e.g. 1.18 to another channel that has normal view mode, the zoom amount remained so the new channel was heavily cropped. This issue effectively made it impossible to use custom view mode on any channel since you had to manually reset the zoom amount after changing back to a "normal" channel.

paging @opdenkamp