drm/radeon: Move radeon_align_pitch() next to dumb-buffer helpers

Move radeon_align_pitch() next to its caller in the dumb-buffer
code. Removes the only dependency on the radeon's fbdev source file
that is not related to fbdev emulation. No functional changes.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>