meta: Use _mesa_get_format_bits() to get the GL_RED_BITS

meta: Use _mesa_get_format_bits() to get the GL_RED_BITS

We currently get red bits from ctx->DrawBuffer->Visual.redBits
by making a false assumption that the texture we're writing to
(in glCopyTexImage2D()) is used as a DrawBuffer.

Fixes many failures in gles3 Khronos CTS test:
copy_tex_image_conversions_required

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>