fbdev: msm: fix ret value while translating to mdp format

Fix return value for error case while converting from
grayscale to mdp format, use integer instead of
unsigned int.

Change-Id: I5b5b529bfab566938a6c55844cd18813fad400b7
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>
Signed-off-by: Vinoth Jayaram <c_vinoja@codeaurora.org>