drm/msm/sde: Create generic function for resource allocation

Current code has two different functions to handle the
resource allocation for continuous splash and for the
commit path. This change optimizes the code by
creating a generic function that can be used by both paths.

Change-Id: I6c756bbfd7b49f9c50f6ae918f2605004127c49f
Signed-off-by: Nilaan Gunabalachandran <ngunabal@codeaurora.org>