egl: deduplicate allocations of local buffer over each platform backend (v2)

egl: deduplicate allocations of local buffer over each platform backend (v2)

platform_drm, platform_wayland and platform_android have similiar local buffer
allocation routines. For deduplicating, it unifies dri2_egl_surface's
local buffer allocation routines. And it polishes inconsistent indentations.

Note that as dri2_wl_get_buffers_with_format() have not make a __DRI_BUFFER_BACK_LEFT
attachment buffer for local_buffers, new helper function, dri2_egl_surface_free_local_buffers(),
will drop the __DRI_BUFFER_BACK_LEFT check.
So if other platforms use new helper functions, we have to ensure not to make
__DRI_BUFFER_BACK_LEFT attachment buffer for local_buffers.

v2: Fixes from Emil's review:
   a) Make local_buffers variable, dri2_egl_surface_alloc_local_buffer() and
      dri2_egl_surface_free_local_buffers() unconditionally.
   b) Preserve the original codeflow for error_path and normal_path.
   c) Add note on commit messages for dropping of __DRI_BUFFER_BACK_LEFT check.
   c) Rollback the unrelated whitespace changes.
   d) Add a missing blank line.

Signed-off-by: Mun Gwan-gyeong <elongbug@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>