drm/tilcdc: Correct misspelling in error message

This error message will be printed when a FIFO underflow irq has
triggered. Since this happens sometimes and the error message will be
displayed on the console, it should have a correct spelling.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Signed-off-by: Jyri Sarha <jsarha@ti.com>