egl: Soften several HAVE_DRM_PLATFORM to HAVE_LIBDRM

egl: Soften several HAVE_DRM_PLATFORM to HAVE_LIBDRM

To fix build when libdrm is not found,
commit a594cec7e3ef275c386054127a357110a19dd823 did put several
parts of egl code under #ifdef HAVE_DRM_PLATFORM.

HAVE_DRM_PLATFORM means the egl drm platform is being built.
What should have been used instead is HAVE_LIBDRM.

At a few locations, the HAVE_DRM_PLATFORM introduced
have already been replaced by HAVE_LIBDRM, this patch
replaces the remaining occurences.

This patch makes for example EGL_EXT_image_dma_buf_import
be advertised by egl under x11 when the drm egl platform
is not built, whereas previously it required the drm egl
platform to be built.

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>