MdeModulePkg PCD: Avoid DynamicHii PCD set to override other values

When Hii variable is not present and if we try to update a variable offset
with some value, we are creating a new variable by Zeroing all the variable
offsets except the one which we are trying to update.
This will override all the other variable default values which are
programmed as a part of initial PCD definition.

DXE PCD driver could be enhanced to combine the DynamicHii
PCDs(related to same variable) default values and only update the offset
PcdSetXXX want to set, then set the combined values to variable.

Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>