graphics/drm-kmod: Fix dependency on firmwares

Don't overwrite RUN_DEPENDS, add to it. Also allow all architectures
supported by the concrete drm port, and IGNORE otherwise instead of only
depending on firmwares.

While here, indent .if/.elif/.endif correctly.

Approved by: x11 (manu), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D36479