drm/msm/dsi: fix irq_of_parse_and_map() error checking

The irq_of_parse_and_map() function returns zero on error.  It
never returns negative error codes.  Fix the check.

Fixes: a689554b

 ("drm/msm: Initial add DSI connector support")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/557715/
Link: https://lore.kernel.org/r/4f3c5c98-04f7-43f7-900f-5d7482c83eef@moroto.mountain
Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>