drm/sti: Include linux/io.h for devm_ioremap()

Include linux/io.h for devm_ioremap().

When built on x86_64 w/ COMPILE_TEST=y:
../drivers/gpu/drm/sti/sti_dvo.c:531:21: error: implicit declaration of function ‘devm_ioremap’ [-Werror=implicit-function-declaration]
  531 |         dvo->regs = devm_ioremap(dev, res->start,
      |                     ^~~~~~~~~~~~
../drivers/gpu/drm/sti/sti_dvo.c:531:19: error: assignment to ‘void *’ from ‘int’ makes pointer from integer without a cast [-Werror=int-conversion]
  531 |         dvo->regs = devm_ioremap(dev, res->start,
      |                   ^

Cc: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240408170426.9285-7-ville.syrjala@linux.intel.comAcked-by: Alain Volmat <alain.volmat@foss.st.com>

drivers/gpu/drm/sti/sti_dvo.c

@@ -7,6 +7,7 @@
 #include <linux/clk.h>
 #include <linux/component.h>
 #include <linux/debugfs.h>
+#include <linux/io.h>
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/platform_device.h>