drm/sti: Handle return value of platform_get_irq_byname

platform_get_irq_byname() can fail here and we must check its return
value.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/f4b4f1b2cfa5e302ef7ffad4e3efb0d3147709d3.1510914877.git.arvind.yadav.cs@gmail.com