ARM: OMAP: Fix omap_sdram_size calculation

Last section of DMM is used for trapping tiler unmapped sections.
Corresponding trap_size should be deducted from total SDRAM size
only if trap section is overlapping with available SDRAM
based on DMM sections. Fixing the same.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>