kgsl/drm: Add platform device feature in kgsl-drm.

This patch adds platform device feature in the kgsl-drm driver.
When we support IRQ feature in kgsl-drm. we need to change to platform
device driver.

Change-Id: I8158fcab1f9b9c1ea50255b524091dd6b8bb8548
Signed-off-by: Eunchul Kim <chulspro.kim@samsung.com>
Signed-off-by: Mathew Karimpanal <kjmathew@codeaurora.org>
(cherry picked from commit 3a5c2741e7eae00f22f25a9205bbd410307e3653)