intel: Fix segfault in intel_texsubimage_tiled_memcpy

intel: Fix segfault in intel_texsubimage_tiled_memcpy

The function segfaulted when a game called glTexSubImage2D on a texture
with internalformat/format/type = GL_SLUMINANCE8/GL_BGRA/GL_UNSIGNED_BYTE.

The function only supports MESA_FORMAT_ARGB8888 and returns early if it
detects an unsupported format. Clearly, its detection condition was
insufficient. This patch fixes it to explicity check for
MESA_FORMAT_ARGB8888.

Note: This is a candidate for the 9.0 branch (fixes 413c491).
Reviewed-and-tested-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Paul Berry <stereotype441@gmail.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>