intel: Always allocate miptrees from level 0, not tObj->BaseLevel.

intel: Always allocate miptrees from level 0, not tObj->BaseLevel.

BaseLevel/MaxLevel are mostly used for two things: clamping texture
access for FBO rendering, and limiting the used mipmap levels when
incrementally loading textures.  By restricting our mipmap trees to
just the current BaseLevel/MaxLevel, we caused reallocation thrashing
in the common case, for a theoretical win if someone really did want
just levels 2..4 or whatever of their texture object.

Bug #30366