board: ti: am62x: evm: Update function calls for splash screen

Use spl_dcache_enable, in place of setup_dram, arch_reserve_mmu to set
up pagetable, initialise DRAM and enable Dcache to avoid multiple
function calls.

Check for CONFIG_SPL_VIDEO in place of CONFIG_SPL_VIDEO_TIDSS to prevent
any build failure in case video config is not defined and video related
functions are called.

Check for CONFIG_SPL_SPLASH_SCREEN and CONFIG_SPL_BMP before calling
splash_display to avoid compilation failure.

Signed-off-by: Nikhil M Jain <n-jain1@ti.com>