backend-drm: remove unecessary ifdef checks

Since commit 28d26483 ("build: bump libdrm requirement to newer version
(2.4.83)"), all supported libdrm versions provide modifier formats,
atomic API and blob formats. Remove ifdef checks (HAVE_DRM_ADDFB2_MODIFIERS,
HAVE_DRM_ATOMIC, HAVE_DRM_FORMATS_BLOB) to improve the code and make it
simpler.

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>
3 jobs for bump_libdrm_req in 1 minute and 31 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Container Prep
passed #1006972
container_prep

00:00:26

 
  Build
passed #1006973
build-native-meson-default-options

00:01:05

passed #1006974
build-native-meson-no-gl-renderer

00:00:31