MdeModulePkg Variable: Add some missing changes for 9b18845

MdeModulePkg Variable: Add some missing changes for 9b18845

To improve performance 9b18845a4b4cd1d2cf004cbc1cadf8a93ccb37ea
changed the code which read from physical MMIO address to read
from memory cache, but it missed some places that could be updated
the same away for performance optimization.

The patch updates these places as supplementary.

I found them when updating code for
https://bugzilla.tianocore.org/show_bug.cgi?id=1323
Merge EmuVariable and Real variable driver.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>