OMAP2PLUS: DSS2: FEATURES: Fix usage of dss_reg_field and dss_clk_source_name

The structures dss_reg_field and dss_clk_source_name have enum members which
specify the register field and the clock source respectively. These members are
not used to choose the correct result in the corresponding feature functions.
Remove these members and change the features array declaration to incorporate
these enums.

The structure dss_clk_source_name without the enum member is just a pointer to
an string. Remove the structure and use a character pointer directly.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>