mesa: add GL_RED, GL_RG support for floating point textures

mesa: add GL_RED, GL_RG support for floating point textures

Mesa supports EXT_texture_rg and OES_texture_float. This patch adds
support for using unsized enums GL_RED and GL_RG for floating point
targets and writes proper checks for internalformat when format is
GL_RED or GL_RG and type is of GL_FLOAT or GL_HALF_FLOAT.

Later, internalformat will get adjusted by adjust_for_oes_float_texture
after these checks.

v2: simplify to check vs supported enums
v3: follow the style and break out if internalFormat ok (Kenneth)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90748
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>