fbdev: c2p: Fix link failure on non-inlining

When the compiler decides not to inline the Chunky-to-Planar core
functions, the build fails with:

    c2p_planar.c:(.text+0xd6): undefined reference to `c2p_unsupported'
    c2p_planar.c:(.text+0x1dc): undefined reference to `c2p_unsupported'
    c2p_iplan2.c:(.text+0xc4): undefined reference to `c2p_unsupported'
    c2p_iplan2.c:(.text+0x150): undefined reference to `c2p_unsupported'

Fix this by marking the functions __always_inline.

While this could be triggered before by manually enabling both
CONFIG_OPTIMIZE_INLINING and CONFIG_CC_OPTIMIZE_FOR_SIZE, it was exposed
in the m68k defconfig by commit ac7c3e4f ("compiler: enable
CONFIG_OPTIMIZE_INLINING forcibly").

Fixes: 9012d011 ("compiler: allow all arches to enable CONFIG_OPTIMIZE_INLINING")
Reported-by:  <noreply@ellerman.id.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927094708.11563-1-geert@linux-m68k.org