mesa: Add GL_BGRA_EXT to the list of GenerateMipmap internal formats.

mesa: Add GL_BGRA_EXT to the list of GenerateMipmap internal formats.

The GL_EXT_texture_format_BGRA8888 extension specification defines a
GL_BGRA_EXT unsized internal format (which is a little odd - usually
BGRA is a pixel transfer format).  The extension is written against
the ES 1.0 specification, so it's a little hard to map, but I believe
it's effectively adding it to the table used here, so we should allow
it here as well.

Note that GL_EXT_texture_format_BGRA8888 is always enabled (dummy_true),
so we don't need to check if it's enabled here.

This fixes mipmap generation in Skia and ChromeOS.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
References: https://bugs.chromium.org/p/chromium/issues/detail?id=630371
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reported-by: Stéphane Marchesin <marcheu@chromium.org>
Cc: mesa-stable@lists.freedesktop.org