fbdev: msm: remove check for rgb source format

CSC bit is enabled for YUV format only, so no need to in-validate
if the source format is changed from RGB to YUV.

Change-Id: I5e0685f50cf3ce7a0d7a08b545cf35f1c303e219
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>