libdrm, freedreno, intel, nouveau, radeon: add Makefile.sources

Will be used to consolidate the required sources lists as well as the
install-able headers. This is turn will help us to avoid the
duplication with the upcoming Android build support.

v2: Rename the headers variable to *_H_FILES.
v3: Rebase on top of symbol visibility patches.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>