mesa: allow half float textures based on ARB_half_float_pixel

mesa: allow half float textures based on ARB_half_float_pixel

In targets that support half float textures but not float textures (so
without ARB_texture_float), the previous logic did not allow for
enabling half float texture support in desktop OpenGL.
OES_texture_half_float is only valid for OpenGL ES 2.0 contexts, so
include ARB_half_float_pixel in the logic to cover OpenGL too.
Remove _mesa_is_gles3 from the check since in case of a gles3 context,
OES_texture_half_float is already assumed to be enabled.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8020>