media: dvb: dvb-core: Fix memory leak in DVR buffers

When DVR buffer mode changes between internal/external, the DVR
buffer pointer gets updated with the new allocated memory.
This overrides the existing pointer to the memory allocated
previously. The code was changed so that when switching between
internal and external modes the currently allocated memory
is released.

Change-Id: I819e42120d50ac4aa7f152ba6fff8269d858cfab
Signed-off-by: Hamad Kadmany <hkadmany@codeaurora.org>