drivers: simplify main build file

drivers: simplify main build file

As with the library meson.build file, we can use the "continue" keyword to
reduce the level of indentation used for the majority of the build file.
Since we are changing the whitespace indentation level, we also update the
body of the foreach loop to use the meson standard, 4-space indentation.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>