msm: kgsl: Remove unused system header file directives

Remove directives containing socinfo.h, board.h and msm_iomap.h
since these are no longer used within our driver.

Change-Id: I1bef6525fe6a1ada1c3d68e77d220ab9f8ac6b3e
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>