msm: mdp3: Correct RGBX_8888 source format configuration

For a layer with RGBX_8888 format, the DMA source format
was not getting selected correctly due to which wrong bpp
value was used while configuring DMA buffer address. This
caused screen content to be stretched for RGBX_8888 single
layer usecase. Add RGBX_8888 support in get_source_format
function to address this. Add same support for MDP_XRGB_8888
and MDP_BRGA_8888 formats also.

Change-Id: I1ad22908c1f4c5efddc0cbe5406b2215c0b83e47
Signed-off-by: Arun kumar <akanak@codeaurora.org>