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: I07690cc3aa86827114419b1d47ee40f47338766b
Signed-off-by: Hamad Kadmany <hkadmany@codeaurora.org>
Signed-off-by: Neha Pandey <nehap@codeaurora.org>