mesa: add missing RGB9_E5 format in _mesa_base_fbo_format

mesa: add missing RGB9_E5 format in _mesa_base_fbo_format

RGB9_E5 should be accepted by RenderbufferStorage if the
EXT_texture_shared_exponent is exposed. It is left to the
implementations to return GL_FRAMEBUFFER_UNSUPPORTED_EXT
when checking the framebuffer completeness if they do not
support rendering in this format.

Discussed in:
https://github.com/KhronosGroup/OpenGL-API/issues/32

This fixes KHR-GL45.internalformat.renderbuffer.rgb9_e5

v2: Added more info to the commit message (Antia)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Antia Puentes <apuentes@igalia.com>