kernel/drm: In asm/io.h, improve the iowriteN() macros.

kernel/drm: In asm/io.h, improve the iowriteN() macros.

The main issue were the trailing semicolons in the definitions.
These prevent usage in braceless if/else, for example:

if (blah)
        iowrite32(...);
else
	...

The resulting double semicolons after preprocessing lead to the
closing of the if (), resulting in an "'else' without a previous 'if'"
error.