ShellPkg/Dp: make sure memory is freed before exit

Run dp command now:
Firstly it will get performance records from FPDT and then
parse the DP command. And if encounter invalid parameters,
it will exit directly. Thus the performance records got before
are invalid. And what's worse is that the memory allocated in
getting performance records phase is not freed.

This patch update the code to parse the command firstly and
then get the performance records. And make sure that all the
clean work has been done before exiting.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>